Bracebridge and Huntsville was served for some time. I know, it used to stop there 20 years ago. There has been a station since the 1880s/90s.
www.uniquemuskoka.com
If I had unlimited money and unilateral control, I would have the regular Northlander running everyday.From May Long till Thanksgiving, a separate train would run between Toronto and North Bay for the weekends. I would time it so that you could get to Toronto on the last day of the weekend/long weekend at are reasonable time.
As far as slots in Union, I would wiggle them in between the via trains.