Being hidden doesn't put anymore strain on the server than being online does. When the location request is called, all it has to do is respond with "Hidden" instead of "Online, at $currentview". And phpBB still will tally up the hidden users, even if there aren't any, in which case it returns a 0.
A MOD for this would put more strain on the downstream of the client and the upstream of the server, but it would propably help out in the long run.
Happiness is not about being perfect.
It is about seeing beyond the imperfections.