The options might vary according to where you are but a couple of things come to mind.

No Doubt's comment (above) on providing two services is certainly possible, but another possibility is what we used to do in the '60s - an interdenominational service. There's probably even some way of including communion for both protestant and r.c. camps...

On the Constitution front, I'd be interested to know your legal status. If you're a registered charity I would think that including any activity which excludes members on the basis of religion would not be looked upon favourably by the Charity Commissioners (or whatever you have where you are).

sian