I usually am able to elevate my foot a little during flight by propping it up on my underseat carry on bag but it's not fully elevated. When I did fly first class that one time there was definitely more room but the bag definitely still doesn't get it fully elevated. Might help a little though.

Also forgot to mention before that I am always the last off the plane too. I gate check my walker so it is waiting for me when I get off the plane so waiting not only makes sure I don't get bumped by those anxious to get off the plane but the walker is always there by the time I get off too.

If you were to take a really late, ovenight flight then there would be a good chance the flight wouldn't be full...meaning maybe you could arrange to have an empty seat next to you which would allow you to put your leg up. I don't know if you can buy two seats...I would think you could because they require it of some people so depending on the cost of a coach ticket it might be worth it for your comfort if that is allowed.