
GM Rednal |
One of my characters recently did something similar, and I would allow it.
1) The character is currently Extraplanar, meaning they fit the targeting criteria.
2) Dismissal is generally inferior to Plane Shift for getting home because of that 20% chance of going elsewhere. Also because it doesn't work on a group.
The spell doesn't say WHERE they go to on their home plane (and the Material Plane is 99+% empty space), but I've always run it as "back to the place they last left that plane from, or the safest place near that point". This helps to prevent weird offensive exploits.

Wheldrake |

That sounds like a very clever use for a spell, and a cool way for the player to get out of an almost certainly fatal situation.
As a DM, I wouldn't let him pop back to the same spot he just left, though. I would think banishment would be at least as inaccurate as plane shift, so 5d100 miles in a random direction (not in solid rock or in the air, but on a habitable surface) might be appropriate.
Any time players can find cool ways to survive almost certain death, that also seem to conform to the RAW, the DM should let them get away with it.

GM Rednal |
...To clarify one of my earlier points, I still feel Dismissal (and similar powers) should send people back to the place they left their plane from. Note that this isn't always a good thing, because sometimes that's not where you want to go. If you used plane shift to escape a situation, for example, you might not want to Dismiss yourself back into it.
In addition, I try to avoid adding extra effects to spells beyond their text. There's already a set chance of failure - 20% to go to another plane entirely, which could be quite hostile to you. Negative Energy Plane, anyone? I don't think there needs to be an extra, unwritten accuracy penalty on top of that, since players are already taking a major risk - potentially resulting in death, if they don't have some kind of planar adaptation ability - by using Dismissal on themselves.