The problem you have with placing a filter in the focuser base itself would be the eyepiece hitting it when you have an eyepiece requiring much in-focus. I know Moonlite makes a focuser that options a filter swing-in/swing/out feature. The website says it can't be used if your OTA requires their universal base. It only works if have a flat focuser board. I've tried every way in the world to have a focuser slide inside the tube but the bottom of the focuser draw tube will hit it with very much in-focus.
I have a 2" Moonlite CR focuser. IT IS AWEOME! I also use the ES HRCC and just have to take it in and out to screw in a filter. I can also reach inside the tube to screw in a filter into the HRCC tube. My HRCC is exactly the same depth as my drawtube but I can still reach in and crew in/out filters. I've been advised that placing a filter in the OTA may introduce reflections in the eyepiece. I don't know how or why. Something about the distance?
Anyway, I'd say look at Moonlite focusers. Look for their filter swing option. Again, I cant use is because the universal base is in the way but if you have a flat focuser board that won't require the universal install kit, it'll work fine. You just have to know how much range you'll need with your eyepieces and buy the correct draw tube. They also have filter slides that go inside the tube, but the drawtube might hit it. Maybe give them a call. They've seen it all and can guide you best. Good luck.