I need to address something that I probably should have brought up before.
Creatures that fall into a creeping pit, either because they were attacked by it or because they could not see it, are not grappled. They simply fall in, as the pit itself is just like any other hole in the ground (except that it moves). I intended to mention that we needed to put a note in there about how to escape (a DC 15 Climb or Jump check, perhaps) but I kept forgetting.

if this means that the attack should not be called Engulf for reasons of clarity, then we need to call it something else.
Therefore, since there isn't really anything holding a victim in there except gravity, and possibly injury, it's relatively easy to escape. Of course, anyone escaping will be attacked again, but that is beside the point. The idea is to get away (if you can) before the hourly astral portal comes up again. If you are stuck being unable to get out of the pit for long enough to be unable to avoid the teleport, that is a shame. If you've been trying unsuccessfully for several minutes to get out of a 10-foot deep pit, I don't think that you will suddenly gain enough leaping ability to jump out when the saving throw comes along.
Therefore, I have to say that I think Reflex is the least sensible of the three. Since the effect of being sent to the astral plane is a teleportation effect, like plane shift, I think a Will save like what plane shift has makes the most sense to avoid being pulled along by the magic. Fortitude, by process of elimination alone, would be the second-best choice.