I already compile custom firmware formy WNDR3800 in vmware. I noticed the Belkni_F9K_1115_v2 is already supported since 15.05 and I have a few Belkin_F9K1123_v1 lying around.
How would I go about figuring out if it's possible to compile OpenWRT to it? It looks like a chips are already supported. Firmware on the website is 7.6MB and the unit has 64MB ram.
I don't know where/ why you get that perception, but they aren't.
The whole target SOC isn't supported at all (search this forum for 'lexra', if you want more details), neither in OpenWrt, nor upstream linux, binutils, gcc, musl, ... Adding support for this arch is of course 'possible', but a lot of work - and once there you'd have to deal with getting the wlan drivers to work in AP mode and using contemporary hotapd version, neither will be easy.