You could try to calculate a set of bounding boxs that correspond to func_respawnroom.
How are these brush entities calculated as being internal vs external? Good question.
Alternatively, if we believe bl4nk and pheadxdll, then around September of last year there was a CFuncRespawnRoom :: PointIsWithin(Vector const&) function which could be used to test if a point were within a given func_respawnroom entity.
The signature for it given in
http://forums.alliedmods.net/showthread.php?t=102731 may no longer be valid and is not part of that extension so it was likely not maintained.
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