Mosfet Regulator heating issue
Heat is not the problem. According to other things I've been told on this site, MOSFETs suffer a thermal runaway problem (hot spots on the silicon) when they are used as voltage controlled resistors.

Is it okay to use a MOSFET in its resistive region with a heat sink?

If you use a linear regulator which is made for the purpose, you will avoid the hotspot problem, but will still need a heat sink. However, in my cursory search, I couldn't find anything that could handle 50 V in and which would provide 500 mA output. The LM317 or LM217 might be able to handle it.

If you have your heart set on making a transistor controlled circuit using feedback, you can use a BJT (Darlington?) instead of MOSFET, however I am hesitant to give more specific advice since I'm a little out of the electronics game these days. You will need a heat sink for sure.