Clarification: the coppermine devs usually don't develop nor maintain bridge files for exotic third-party apps. We expect bridge files to come as contributions from the corresponding communities - in this case, from the xmb community. If the community will provide a working bridge, that's fine - we'll include it with future releases. If the xmb community doesn't, we can't help this.
The bridging ability of coppermine is an extra feature that coppermine used to have first (before all of the "competing" gallery applications came up with a similar thing, more or less). Coppermine has been designed initially with bridging in mind at a very early stage.
However, the coppermine devs simply can not keep up with the progress of development for all the bridging apps: we'd need to maintain way to many testbeds and we'd have to keep way to many applications up-to-date. That's something we simply can't do.
Seen in relation to the popularity of phpBB, xmb is an exotic application. We don't have the time to look into the bridge file for xmb, sorry.
Applications that could potentially be bridged with coppermine go by the thousands: in fact, every application that has it's own user management could be used for bridging purposes, provided that someone comes up with a bridge file. There are constantly requests by users to see particular bridge files developed. The answer remains the same: the bridge file needs to come from the community. Strong communities will provide one. Not-so-strong communities can't, which is a pity, but we can't help.