It sounds fine to me... then again I have never worked with flaws so that is just my gut talking... haven't even looked at the ones in the SRD recently...

If you want it to scale a bit, make it trigger at increased DCs when grappled by a creature more than 2 size classes or more larger, and/or when swallowed whole... since neither of those are likely to happen at low levels, and realistically are more severe situations than merely being in a narrow corridor it would seem to me to make sense. It is both tighter, and INTENDED to cause harm.