Does it work in the 'in-game' forums or do you mean these forums?
Only a limited set of the bb-codes are supported: http://i.prntscr.com/5GYCfMJxQgCmTseiLC4UTw.png
as listed when selecting the 'bb-code' button in messages.
These forums are using dedicated forum software so have many more features than those in the game. I'll pass on this to the developers of the in-game software but, as is often the case, it is a weekend so don't expect a comment soon.
As an experiment with my UK keyboard I tried the ALT+numeric keypad route.
ç is ALT-135
and this displays correctly in both the chat window and in messages.
I have no reason to think other Extended ASCII codes won't work - that is those from ALT-128 to ALT-255