Quartz lets you schedule ahead, so you do need enough lead time for the command to execute. That being said, people get a way with fairly minimal lead time (down to a 1/16th note).
If you want to make sure you have enough time you can use Quartz to query how much time there is until the next boundary and make a decision in the BP.
>“I’d need to continually retry until success with slightly different start times, but it would help.”
What is the desired fallback behavior if your deadline is going to be missed? Would using Current Time Relative or Bar Relative work? (i.e. you want to make sure the stem starts on beat 2 of a bar, either this bar or the next).
>" callback from the audio thread that allows me to adjust StartTime to the actual clock at buffer submission."
Out of curiosity, what info would that callback provide? Would this be calling back to your BP or do you have a C++ implementation?