Fontification of the interactive prompt string?

[Marcin Borkowski]

Hi all,

I have an (interactive "cprompt\n") call, and I'd like the prompt to
list possible characters (they define a few options for my function, and
I have good reasons to implement it this way and not with C-u etc.)  I'd
like to have the relevant characters highlighted, like in:

"file: _n_ew, _o_pen, _c_lose"

where the underscores mean that something is e.g. in a different color.

Is it possible?  Bonus points if instead of colors the relevant letter
might be e.g. put in brackets on terminals not supporting colors.

TIA,

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University