We were there over the filming last year and here's what I'll say:
-The day of the castle stage performances filming is okay, the crowds aren't that bad, but be aware that things like the 3pm parade, the Move It Shake It dance party & of course the castle party are affected.
I believe the daytime parade was at 5pm instead, while the others were cancelled completely for the day.
Same goes for the filming of the actual parade.
On the actual parade filming day, the main street/hub will be JAM packed, they'll be escorting you around through the backstage ways to get places. We found the rest of the park to be pretty dead that day, but again, main street was crowded.
And again, things are affected, any character who has an outdoor meet and greet (jasmine & aladdin, anyone who's in town square, tiana, peter, etc) probably will not be doing sets, and if they are, they're super messed up (since they can't be out at the same time as the parade filming is happening)
As for taking part in the filming, there are cast members and guests who sign up or something that will be the ones on the street that you'll see on xmas day, you'd be a couple rows back, stuffed in the crowds. We arrived at the park at rope drop, and were there til just past noon, we were later boarding our plane that afternoon when NPH said that they had JUST wrapped filming the intro. The film industry is quite boring, they'll take 5 hours to film 5 minutes of footage, and it'll be the same thing over and over again.
I'd say if you're going to MK for the rides, then yeah, it's fine, otherwise, I'd go somewhere else!