Bazz - whilst your singers might call out the songs in any order, is it always the same songs? (accepting there may be 30 or 40 of them)....cos....
That being the case you really should preprepare what it is you are going to back them with for each song and save those registrations to your SD Card including intros and endings - for immediate call up.
If they are standard hit songs I might well have specific rhythms including the right intros and endings to help you out...
Rog