IanC, you are obviously misinformed.

There are always religious rituals one doesn't participate in when visiting a church or temple or synagogue of another religion. In this case, the religious ritual that people don't participate in is communion. However, not taking communion doesn't exclude anyone, or forbid them from attending. There is nothing to stop a non-Catholic person from taking the communion, of course. No one stands with a gun, checking people's baptism certificates at the altar, after all.

If people wish to have Protestant communion on the day, then they are, of course, free to attend the Protestant church of their choice on Sunday, while the mass is taking place, or before it, or after it. As someone suggested, you could print a list of local churches convenient to the event locations, for the benefit of non-Catholics. But nothing else is necessary, IMO. Nothing.