[plt-scheme] Dealing with UTF8 text (and displaying it)

Dear list members,

I might be wrong but I think that OS X encodes filenames in UTF8 - or 
is it Latin1 instead of MacRoman?

Anyway, I need to display file names and any character beyond the 
normal ASCII range from 0 to 127 displays as garbage in MrEd controls 
such as list-box, editors, menus.

As far as I can see, I'd need to convert Unicode sequences (2 bytes?) 
that have an equivalent in MacRoman [128..255] to MacRoman (or to a "?" 
if no conversion is possible).

Question: I've searched with Google over and over but I couldn't find 
any useful conversion tables. Does anyone have any hints,  insights, 
links etc. that might help me in implementing the conversion?

Related question: Is there any chance that DrScheme will ever support 
UTF8?

Regards,

Erich