Okay, that narrows down the possible solutions somewhat.

The best approach I see now is to have Hijack create a "shadow" device that overlays the disk partitions solely for the purpose of launching executables.

So things that EXEC/EXEC_ONCE launches would be launched from the "shadow" device rather than the actual (in this case) music partitions. The shadow device really just maps things to disk blocks on the music partitions, but without having the actual executables "open" on the music partitions.

That way the unmount/remount will work fine from emplode even with third party apps running. The fsck shouldn't have any effect on those apps, unless it actually relocates the data segments of the executables themselves -- which should not happen, but if it does, the executables will eventually just crash in all probability. No big deal.

Third party apps may have to be more careful with files that they open on the fly --> if they keep files open after startup then this will prevent Emplode from unmounting partitions, but I don't think this is an issue for many of them. Most apps are only really active when selected from the Hijack menu, something that generally is not done while sync'ing. GPSapp may be an exception there.

??