Vesa modes operates on system memory (all bitmaps are stored there), which, in read operation, is much much much faster than the video memory accessed by CPU.
For a start just hack vesa.hidd from x86 native (arch/i386-pc/drivers/vesa.hidd/*).
Add some code which setups some visible area in chipram and then modify vesaDoRefreshArea() to do a chunky to planar conversion into those bitplanes.
You could probably "patch" in emulation in the x11 drivers setattrs function.. see
aHidd_Gfx_DPMSLevel .. and .. vHidd_Gfx_DPMSLevel_On vHidd_Gfx_DPMSLevel_Standby vHidd_Gfx_DPMSLevel_Suspend vHidd_Gfx_DPMSLevel_Off