Well, I'm pretty certain that I'd use Qt for a platform-independent GUI - it looks pretty native on all platforms, including OSX.
Are you sure there are no Windows versions of the VSTs your are using?
I would be willing to help with the gui. I tend to know how to use QT4 designer and can work with c++, so wrapping a .ui file for each dialog would be a piece of cake, jut it would take a while, btw I also wish to add support for a new kind of tracker module (SMPS/68k) which is the format used in Sonic the Hedgehog for sega genesis. the module format is very well known.
here is an example of one:
00 A3 06 03 01 03 00 30 00 00 00 3E 01 E0 00 47
01 E0 00 5A 01 E0 00 7A 01 E0 00 8A 01 E0 00 93
C0 EF 00 E0 00 9B C0 EF 00 E1 00 9B C0 EF 00 E0
81 1A 80 82 82 81 81 80 80 81 80 F9 00 2F EF 00
82 82 82 80 F9 00 3D EF 01 98 98 31 9A 80 93 92
93 93 92 80 98 98 10 F9 00 46 EF 00 D1 D1 D1 D1
80 D1 D1 D1 D1 80 F9 00 59 EF 02 87 87 87 87 80
EF 00 97 97 97 97 80 F9 00 59 EF 02 81 81 80 82
80 81 80 80 82 80 81 F9 00 79 EF 00 92 00 90 90
F9 00 89 9F 80 9F 80 F9 00 92 81 00 81 81 81 81
F9 00 9A FB 83 00 1F D4 01 1A 83 D8 1F 1F 32 88
00 00 00 32 18 63 32 00 3F 38 63 3F E9 83 00 1A
D4 01 1A 03 D8 1F 1F 32 88 00 00 00 32 18 63 32
00 3F 38 63 3F 93 83 00 1F D4 01 1A 83 D8 1F 1F
32 88 00 00 00 32 18 83 32 00 3F 38 63 3F
sure it doesnt provide a decent tune bot more an endless cacophony but it is an example of a SMPS module in its completeness (with 5 FM channels + a pcm drum channels + 3 PSG tone channels)