I'm writing down this info in case someone else would need it.
After a couple days of fiddling I can more or less make some conclusions:
If you have a system with LX800\CS5536, have access to BIOS chip and nessecary tools\skills, you may try cross-flashing it with firmware from another similar LX800 MoBo.
It should work in most cases provided:
1) CPU and companion chip match.
2) Super I/O chip also matches or is compatible register-wise
3) If RAM chips are soldered, they have the same characteristics (often there's no SPD chip either and memory init comes from BIOS code)
4) Peripheral devices like LAN\PCI\VGA should work after swapping corresponding ROMs,editing PCI Tables or changing default BIOS setting for panel\resolution (Award\AMI)
5) Proprietary stuff like FPGA may or may not work, but the system should boot in any case. May also be custom PLL circuit, but I have not seen that yet.
This can be considered as a workaround for MoBo's with restricted BIOS. There's also a coreboot build for LX800, but I've not tried that.
Of course, there may be caveats. Like a non-working temperature sensor, which forces Linux installation to halt, because it reads "255" value.
My exprements mostly involved Fujitsu-Siemens Futro A220 and Advantech PCM-9375
The former successfully worked with Insyde\AWARD\AMI. W83627HF is a very common I/O chip.
The latter is a real pain, as it uses SCH3114 for Super I/O. Not many manufactures did use it. But on the other hand, there's no real need for flashing it as Advantech uses standard Award BIOS.
As for downclocking - 133Mhz is the maximum I could get, though not very stable.
This CPU cannot clock lower than memory. And memory cannot clock lower than that, because DDR DLL does not work below 125Mhz (according to JEDEC datasheet)
I tried disabling DLL (Delay Locked Loop) through MSR registers, but without any results so far. System just freezes or crashes.
Other than that - if BIOS allows you to set Manual CPU\RAM speeds beforehand, ignoring H\W straps, you can then change those settings on the fly through MSR address 4C000014h (editing MSR with AUTO setting in BIOS makes system freeze)
So, it would not be very difficult to make programm like cpuspd to set LX800 multipliers.