I looked up what R Soloveichik writes about goyishe speakers. He forbids any discussions related to theology, religion, but absolutely advises cooperation between religious people of different faiths on social and world issues, not just on issues relevant to Jews.
On relating to other Jewish groups, he rejects an invitation to a dinner in 1960s honoring a Rav/Rebetzin that he knew well personally combined with an opening of a new synagogue. He explains that people who build the synagogue should be proud of their achievement, and that he would come to the dinner in honor of the Rav, but he can’t attend because the synagogue will most likely have mixed seating (during prayers, not during the dinner). So, this would be a red line – approving halachically wrong practice.
This also does not mean, of course, that he will favor Jewish students listening to a pastor. In abother letter in 1950s, he disapproves an interfaith chapel at Cornell explaining that students will learn to see such an arrangement as normal and take this notion to their communities.