The following is the results of both blind googling and brute forcing the keyboard space. If this is incomplete, please let me know so that can be fixed.
You can also type a forward facing accent on most arbitrary characters by typing it and then pressing Alt-Shift-e. Circumflêxes can be done postfix with Alt-Shift-i too. Thís dóesńt work on every letter, unfortunately. However it does work for enough of them. Not enough for Esperanto's `ĉu` however.