Is there a way to use an alt / option key as a meta key but still be able to use it to make some characters which need it?
For example, in my l
Aquamacs supports that, at least for my (french) keyboard. Once Aquamacs is installed (via drag'n'drop), you can go to Options -> Option, Command, Meta keys and choose an appropriate setting for your keyboard (in my case, ...Meta & French). I am not proficient with Emacs, but all the useful combinations I have tried seem to work ([, {, |, and so forth).
Supported keyboards, according to the menu items as of version 2.1, are :
I would guess the Aquamacs team would be grateful for any contribution, though, should your keyboard not appear in the list.