The problem with the 'masterclass' sort of workshop is that it often becomes little more than an intimate 'chance to meet' event, with the guest doing a few turns, explaining a bit about the tuning and phrasing, nattering on in a pleasant and discursive fashion and maybe leading some choruses. I've been to plenty and they can be very enjoyable, but...
A proper workshop is hard graft. It has to be fully participatory and focusses on the performance of those attending, not the guest - and the guests needs to be more than simply a good performer. Sidmouth has had some excellent examples over the years, with good voice coaches really helping to bring singers on.