Masking, by canon, doesn't do that...

Reference?

Thank you for the reference.

In which case I would suggest developing an enhancement or variation on the masking technique that would allow you to re-shape your astral form to whatever you want it to be. After all, it is just for appearances as it would possess no additional magical or protective value. Call it a specialization of the stealth skill if you want.

Minor nitpick: They appear in an "idealized" form on the astral => Their astral body (which is something different than their aura) doesn't necessarily look like the true animal while they are in human form. "Idealized" can also mean that their astral body looks like humanoid with an animal head and similar distinguishing features.
Only in animal form their astral body will appear as their true form.
Apart from that they still have an visible aura that can be assensed.

In order to understand the masking technique you also have to remember some important things about the astral and how things are represented there:
1. Any object (living or dead) appears as a greyed "picture" of itself, with all features of the physical body
2. All living beings and magically active things additionally have a colored aura that surrounds that "picture" and can be assensed. If a living being is killed or a magically active object is destroyed (e.g. a focus' enchantment is destroyed in astral combat) the aura goes away, but the "picture" of the physical body remains.
3. Astrally active entities (like natural dual beings or perceiving mages) gain an astral body that practically overlays the "picture" of the physical body. That astral body is colored as well. p. 161 SR3 describes that astral body as an exact replicate of the physical body. For shifters however there's that exception that I tried to explain above.

Now what can be achieved with the masking technique?
1. It interferes with assensing => it makes some changes to the aura that makes it look mundane and in the case of shifters explicitly allows them to look "human"
2. It allows the magic user to make his astral body invisible => Although the user might be astrally active all a perceiver can now see is the picture of the physical body and the aura, thus it looks as if the person in question is currently not astrally active (although touching that person on the astral will reveal that this actually is not true).
3. Technically it's even possible for a projecting mage to make his astral body invisble in this way. The problem is however: His aura will still be there. And without the picture of a physical body to explain the existance of the aura for a perceiver, any onlooker will notice that something is plain wrong when seeing a "flying" aura, thus rendering that option more or less useless (as noted in MitS).

I believe that the following, with Masking, can disguise their aura, so as to not reveal their true nature. There are probably more. (I've put four on the list now, and will add more as they are pointed out to me.)

Nitpick: Disgusing their aura is not the same as disguising their true nature. Masking cannot IIRC allow someone to make their aura unrecognizable. IE: Bob will always be bob, but he can mask the fact that he's magically active, or a shapeshifter... ect... nothing about the aura really gets masked with masking except secondary details, except in the case of the shapeshifter which is specifically stated they can do that.

That's the part I was talking about. The probelm is: Aura and astral body are two seperate things. Masking allows to alter auras to a certain extend, but they cannot be totally hidden with that. Astral bodies can be made "invisible", but they cannot be altered in shape.
When looking at the assensing table you'll notice that auras do not provide informations on species, race or gender. So unless the aura itself doesn't have a shape that automatically identifies a being as not "human", masking will make a shifter look like a mundane human once he has dealt with his idealized astral body.
Luckily enough masling explicitly allows to hide the astral body from astral perception. So a shifter capable of masking could hide is idealized astral body from astral perceivers and thus would remove all "optical" signs that would normally give away his true nature. All that could be perceived would be the reflection of it's physical body (now human), sourrounded by a aura that can even be masked to look mundane.

Of course you could interpret that sentence to mean that they can actually alter their astral body with masking, but that would be rather inconsistant in comparison to normal mages who cannot alter their astral body ...

[quote]I believe that the following, with Masking, can disguise their aura, so as to not reveal their true nature. There are probably more. (I've put four on the list now, and will add more as they are pointed out to me.)

• Dragon: Dual being, same as shifter => hide astral body and in human form no one will notice the true nature unless mask is penetrated
• Shifter ... see above ...
  Flesh-Form Insect Spirit ... Problem there could be that even if blocks his astral body from sight, his physical features could give him away ...
• Vampire (and other HMHVV-I patients): No natural dual being. So all he has to do is to remove the traces of "sickness" from his aura ...
• Nosferatu: Dual being, same as shifter, but additionally need to hide traces of sickness in his aura
• Ghoul: Block out astral body, remove traces of sickness in Aura.
• Drakes: Are dual beings only in their dragon form. Could hide their astral body, but reflection of their physical body would give away their truze nature anyway.
  In human form they do not retain obvious signs, since they aren't dual beings then and thus have no astral body at that time, but their aura can give away their nature. Hide those signs and you're fine ...