The best way out would be if I could enable ConTeXt's UTF-8 regime while running XeTeX in \XeTeXinputencoding=bytes mode, but I haven't gotten that to work yet.
That would mean that you loose the whole range of glyphs & scripts outside of the scope which ConTeXt supports (you would land almost at the level of pdfTeX again). For most european users that might still be something reasonable, but I wouldn't go that way.
maybe mojca has
(little correction to what I wrote in my previous mail) If you were really looking for that part of code - simply replace \expandafter \endinput inside XETEX block in regi-utf.tex with \XeTeXinputencoding=bytes. Then \enableregime[utf-8] will mean that ConTeXt took control over utf instead of XeTeX. From what I understood on the wiki, it probably used to be that way at the beginning, but then Hans changed his mind and decided to ignore \enableregime[utf] completely when processing with XeTeX. Mojca