I've seen some people use a roller bag as a footrest when seated in some exit rows. I think it was an old version of the 777, about 3 years ago, when I was flying DFW-NRT.

If you're going to do it, I'd just get it out and set it up, then act as if it is normal. Much easier to beg forgiveness than ask permission. As long as it doesn't interfere with crew seats. On those long flights, the crew mostly disappears during the flight after the meal anyway.