According to Cirrus Logic 'tm CL-GD5446 Technical Reference Manual ( http://www.tjd.phlegethon.org/gd5446trm.pdf.gz ) - CL-GD5446 Chip (and later - GD546x, GD5480) supports 32-bit color modes. To set these modes you must have a card with 2Mb of ram (qemu has 4Mb) and make a TWO little different registers setup comparing to 24-bit modes.

1. SEQUENCER REGISTER SR7 - Extended sequencer mode [Outport(0x3C4,0x??07)]

 640x480 24-bit mode: 0x15 (sets 24-bit color mode)
 640x480 32-bit mode: 0x19 (sets 32-bit color mode)

2. CRTC OFFSET REGISTER CR13 [Outport(0x3D4,0x??13), bit 8 at CR1b[4] i.e. 0x10 ]

 how to calculate it:

 640x480 24-bit mode: 640 / 8 * 24 / 8 = 240 i.e. 0x00F0
 640x480 32-bit mode: 640 / 8 * 32 / 8 = 320 i.e. 0x0140

 now set it:

 640x480 24-bit mode: 0x0F0 (sets 24-bit color mode)
 set CR13 to 0xF0, set bit4 of CR1B to 0
 640x480 32-bit mode: 0x140 (sets 32-bit color mode)
 set CR13 to 0x40, set bit4 of CR1B to 1

Thats all. I attach a screenshot that demonstrates 32-bit color in qemu0.9.0+winnt4 in non-standard mode using this technique.