I'ved a small roller board in exit rows (A333 and 744) as a foot rest in about two dozen flights now. I usually try to keep a low profile by covering up with a blanket.
99% of the time, I did not have an issue. One FA even commented what a great idea it was!
On my last flight (AMS-DTW), a FA didn't care for my improvised footrest, and made me put it up, so YMMV...