You should be very up front about your desire to pay for the endorsement but don't bother telling anyone until the interview because no one cares until that time. When you get into the interview you need to tell them how you'd be prepared to pay for the endorsement and even for the upgrade if it meant less time as a S/O.
I'm sure those people doing the interview will be so impressed that they'll bump someone off the next course to get you on.