If there was an interdenominational service would you object to that because it discriminated against and excluded agnostics and atheists?

Or, as No Doubt just said, if you were playing Klezmer music, would a service in a synagogue as part of a festival offend you?

If this organisation you were in were barring non-Catholics from membership, or doing things that made it impossible for them to belong, you would be quite right to kick up a fuss. Or if the Mass was conducted in a wash that was designed to stir up sectarian hatred, with some clerical bigot up there preaching hate.

But from what you say, I can't see there is any problem (there may be other things you haven't said that would change that view of mine). Sticking the times of services of all religious denominations might be a good way of indicating a commitment to non-sectarianism. Maybe there might be room for a multi-denominational service as well. (Though you still might get hassles from people who would say yes to those two questions I posed at the start of the post. To which I'd be inclined to say "get a life", which is something I very rarely say.)