You could put a couple of fans outside in the hall directing air into the room. Fans may be noisy, but if they are outside the sound should be mitigated a little at least.
Do you have to have rehearsal in the choir room? Could you relocate to some place that has A/C or is cooler such as a basement?
To participate in the discussions on Catholic church music, sign in or register as a forum member, The forum is a project of the Church Music Association of America.