I'm having a hard time believing this has never been covered, but i can't find a thread. Do wards block access from the metaplanes? The specific thing i'm curious about is whether a spirit (assume this also refers to elemental) can sit on a metaplane and sustain a spell .

Note that this is coming from a SR4 POV, and maybe this is why it hasn't come up in SR3. At least the sustaining part, because SR4 got rid of Exclusive actions (plane switching i believe was Exclusive, so was sustaining). But still it had it should have come into play when tapping an elemental you had on standby while you are inside a unfriendly warded area.

I would tend to say that wards do not block access to the metaplanes and vice versa, as you can burn a service of an elemental to go through metaplane back doors and appear on the other side of a ward.

Within SR4, without Street Magic available, I think you are touching on some of the finer points that may have to wait.

MitS p. 96-98 discuss Metaplanar travel of Spirits, mentioning the fact that Elementals and spirits can travel from the physical plane to their native Metaplane without passing through Astral space. They can use this shortcut to bypass astral barriers such as wards at the cost of one service.

SR3 p. 187 Spell Sustaining--It specifically mentions that the elemental can stay in AStral form while doing this, and since each service is exclusive, it cannot leave the range imposed by the summoning (Cha+Wil+Mag)*10 m. Thus it cannot travel to the metaplanes while sustaining a spell.

Some GMs may rule that by using a Remote Service and giving up all further services, a spirit can be bound to sustain a spell on it's home metaplane. I would personally say no, since both tasks result in the spirit owing no further services, and thus the mage is trying to abuse the magical system and get an extra bonus without paying for it.

The binding of a spell for sustaining in SR3 sustains the spell for a day per pt of force the elemental has, but the elemental is destroyed and cannot be re-summoned in the future. It may be possible to word a remote service to have the Elemental begin sustaining a spell which you cast and then travel back to its home plane and hold it there.

The drawback associated with each task is removed through this type of task though. GM adjudication would be required to determine whether it takes one service, two services, or is simply not doable.