Does it help with using cameras for video footage, or would it be no problem to hold the mirror out of the way for a long time? (I imagine it being held in some kind of tension that wouldn't like long term force on it).
My DSLR had a hard limit of 30 minutes video time, but I suspect this is because of the software issues, not mirror tension per se.
That probably was a tax/duty thing, the EU treats (treated? not sure) cameras that can record over 30 mins or so as video cameras which have higher fees.
Yeah, that's still going on, dammit -- my new D780 has the 30 min. limit.
If it's a Canon DSLR and is supported by the Magic Lantern firmware modification, it will allow you to circumvent this limit that way. It's possible that you may then need to use an external recorder via HDMI to avoid overheating depending on the model etc.