I got my hands on a PT-733A mainboard which has the not-so-common UMC 8890 Pentium chipset. Now I am trying to collect some information on this type of hardware.
Info on the 8890 is ...scarce, even worse than the 8881/6 chipset. Very few drivers and BIOS images exist and absolutely *no* datasheets at all. This is what I have gathered so far:

BIOS: The PT-733A is the one of only two socket-7 mainboards which have the UM8890, the rest is socket-5. The BIOS is an AWARD 4.51PG type from '96, I have dumped it (see attachment). Most other BIOSes I have found are 4.50PG or AMI WinBIOS. Some of them can be transplanted the PT-733A, provided the Super-IO chip is the same.

Drivers:
- There's an IDE driver package on TRW which also contains an IDE Miniport for Win9x and the UM8886BF. However, the PCI id (VEN_1060, DEV_673A) is missing in the .inf-file, so I added it. The driver itself works fine in Win98, though no DMA or bus master support as far as I can see.
- There is no (proper) UMB driver for this chipset, since nobody knows the registers which control shadow RAM access. I had hoped that 8890 would be to some degree register-compatible with 8881, but this does not seem to be the case. I managed to write a small DOS driver (UM890UMB.SYS) which makes some part of the upper memory area writable (C800-D7FF) and thus usable for UMBs. You need an additional UMB driver like UMB.SYS from URAM or HiRAM.EXE. Simply load UM890UMB in CONFIG.SYS first, then the UMB driver, then HIMEM.SYS and everything else. This gives you ~80k of UMB space.
The registers which control shadow RAM access seem to be reg. 5C, bit 0-6 (write protect?, 0 = off) and 5D, bit 2-7 (set to 1) in the 8891 PCI host bridge.

PS/2 mouse: The PT-733A has an unpopulated PS/2 header next to the keyboard connector. It is fully functional, just solder in a 5-pin header and connect the PS/2 mouse as follows (Pin 1 is the closest pin to the keyboard socket):
Pin 1: -> Data
Pin 2: -> n.c.
Pin 3: -> GND
Pin 4: -> +5V
Pin 5: -> Clock
Pin 6: -> n.c.

Problems:
- There is something weird about the IDE chip and/or the BIOS on this chipset. First, It does not support hard drives larger than 8.4GB. I don't know if this is solely a BIOS limitation, since all BIOS images I tried (AMI, AWARD 4.50 and 4.51) all gave me the same compatibility issues. I can use larger HDDs with an XT-IDE ROM, but then Win98 doesn't see my CDROM drive anymore.
Also this chip doesn't seem to like CF cards that much, some work fine ( 8GB industrial type) while others just give me I/O errors on read.
- The chipset seems to be picky about PCI video cards, newer ones do not necessarily work.
Cards which do not work (not booting): Voodoo 3, Savage 4, Kyro
Cards which work: S3 Virge (and older), Riva TNT, ATi Rage Pro (and probably others, I did not test so many cards yet...)

That's all I can think of a.t.m, any further details are highly appreciated.