Earned my multi- rating at BJC years ago, so I don't have any current reports, but, I think McAir and Rocky Mt. Flight School at BJC, Rocky Mountain Metro airport, have Seminoles for multi-training.
As an aside, all other factors being equal, BJC might be a better airport to operate from, simply because APA, Centennial, can be very busy and has a greater potential for burning up a lot of Hobbs time on the ground, though, bigger factors such as where you're staying in the Denver area, scheduling availability, etc. may have more bearing on a decision.