The only hotels Ive seen that have ever blocked the hdmi are at some Marriotts (but none of their sister branches oddly enough). It's the ones that still have that old assed 90's lodgenet sytem.
I friggin hate Marriots, especially before I was a member and they charged for wifi, seriously, wtf? Anyway I remember calling the front desk and they were going to send maintence up but they took forever, probably unclogging toilets or smoking cigs out back, but I ended up figuring it out myself by unplugging the rj45 connector in the back. There's multiple hacks on the net to get around this.