There is no such thing as a 'unicode' file format. Unicode is a standard that defines integers (not bytes!) to glyphs. Unicode can be encoded to bytes as UCS2, UCS4, UTF-8, UTF-8 with BOM, UTF-16LE, UTF-16BE, UTF-32LE, or UTF-32BE.
Winamp 'unicode' is UTF-8 with BOM (since thats the win32 default for utf-8, theological discussion aside), however it will read straight UTF-8 just fine (in my experiments).
If you are asking for comprehensive encoding support ... thats not easy. Mirc uses, by default, UTF-16LE because thats what wchars are in win32. Doing something else would require all the strings you can access through mirc to link in software that either has to be written from scratch or is notoriously hard to link against on windows (or even illegal to link to mirc without drastically changing it's license).