I always seem to forget this one.
To pass F11 or F12 over a serial connection (either real serial or Serial-over-LAN IPMI), it’s Escape followed by ! (Shift+1) or @ (Shift+') respectively.
Note that on a US keyboard ! and @ would be next to each other above the 1 and 2 keys so that would make some vague kind of sense as alternatives to F11 and F12. But it’s literally the @ that matters and since I’m using a UK keyboard then it is Shift+'.