This power works on mundanes, are technomancers mundane for this?

I would say yes. This power is talking about mundane vs awakened, as in normally only awakened individuals can interact with the Astral. Everyone else (non-awakened) would be "mundane" for that purpose.

I would personally rule that the Emerged (technomancers and others with a Resonance attribute) could utilize this, but would lose access to their Resonance while projecting as they essentially gain a temporary connection to Magic.

If SR5's version is anything like based on:
then all residents of the 6th World are divided up into Awakened and Mundanes, alternatively into Magicians and Non-magicians. Whether otaku belong to the former or the latter is largely irrelevant, both categories can be subject to Astral Gateway / Rifts.

I agree with Kiirnodel though that an otaku would lose access to their mojo "as if they were a full magician".

Well, it's not as if there are a lot of things to hack on the astral plane.

But since the otaku's meatsack is slumped in his beanbag back on planet Earth, still within range of the Matrix, I certainly could imagine a player whose character is astrally projecting through a warehouse / office block / etc. saying he'll use his innate matrix sense to track down and bring up a blueprint schematic of the facility to guide his astral travelling. It's a legitimate question. A cybermage who's proactively downloaded the map in advance would have this knowledge, so it's fair to resolve whether an otaku can reactively obtain it on-the-fly. I guess I'd consider allowing it with the +8 TN for overriding the RAS Override for them to concentrate on the Matrix without being Disrupted by their shift in attention.

Or the -6 DP penalty for SR4 & SR5 rules on working in and out of VR at the same time.

Except that while projecting, you don't have access to any sensory input coming in from your physical body.

That may well be the case in SR5, which this thread is about. I do remember in SR3 though:It's just like in Looper where giving people at the body a way of communicating with the remote mind (and vice-versa as long as the projecting character has a scalpel weapon focus). While the quote says the damage is experienced by (and equally lethal to) both partitions of a projecting character it doesn't explicitly state that the character perceives the pain from the wound. I expect they would, however.

I'm not entirely convinced that experiencing the Matrix as an otaku / technomancer is equivalent to simple sensory input from a physical body. It seems to arrive in the awareness or consciousness.