The documentation on the ET4000/W32P STG1702 I can't seem to find anywhere.
But the STG1703 can be found, with entries matching those modes I see explained in WhatVGA (only up to a certain register though, where the documentation abruptly ends).
Oddly enough, I see two interesting modes in there:
Mode 6: 16-bit 5:6:5 direct color (2ψ8-bit input)
Mode 7: 8-bit indexed color (2ψ4-bit input)
Mode 8: 15-bit direct color (2ψ8-bit input)

Those seem like the equivalents of modes 0(8-bit), 2(15-bit) and 3(16-bit), but 2 VCLKs per pixel instead of 1?

Also, what happens during the modes when these extensions aren't enabled (the AT&T 20C490-compatible modes when bit 3 of the DAC command register cleared)? Are they identical in behaviour to the AT&T 20C490 chip?