I don't know any BIOS files to suggest, but the fact that the chip is soldered would make me nervous.
It's annoying that it seems like all OEM boards have soldered BIOS chips. They're more expensive to replace than home-build retail motherboards, so it's kind of weird that they don't bother making them repairable in the event of something as simple as a bad flash.

If the experiment fails, will you be able to desolder the chip? If you can, then it's not such a huge risk at that point, just an inconvenience. Definitely make sure you save a backup of the original BIOS before you try anything.

I've never messed with crossflashing so I don't know which characteristics of the boards most critically have to match for it work, or how good the chances are even if the boards do match. I suppose that some boards could have the same devices connected at different addresses, which would make the BIOS code not find those devices where it expects them to be.

I can solder DIP stuff just fine, but I have little experience with desoldering/soldering SMD stuff. If I feel brave I might solder an SMD socket and a compatible DIP BIOS chip.

Are you sure the Compaq partition needs to be there?
The way I read the docs, if the partition is present, you only need
to press F10 to access the Compaq 'bios setup/change parameters
and diagnostic utilities' that Compaq prefers to place on a hard
drives 'Compaq partition'.
However, you can also have no 'Compaq partition', and instead
place the 'bios setup/change parameters and diagnostic utilities' onto
3 floppy disks - so whenever you want to change the bios/CMOS
parameters or run Compaq diagnostic utilities, you can boot from the
floppy disks to do these things. (Obviously, you would want to make
extra backup copies of these 3 disks, and when running them, you
would make sure you did not let them create any 'Compaq partition'.)
Reference;
ftp://ftp.compaq.com/pub/supportinformation/f … _MMX_Models.htm

Also, you stated it is a "Compaq 5200MMX".
But there were several models, e.g.; Compaq Deskpro '2000 5200MMX',
or '4000 5200MMX' or '4000x 5200MMX', etc.
To be sure you are downloading and using the correct bios for that
motherboard, you need to find the 'COMPAQ (or COMPAD) S/N' and input
that 'Product Serial Number' into this HP (owner of Compaq) webpage;
http://h20566.www2.hp.com/portal/site/hpsc/te … D_op%253Dserial

(The bios setup/change and diagnostic utilities, and programs to
create those utilities onto floppy disks, can be downloaded from HP.)

As it is not clear what model you have, it is not clear what the max
size of hard drive is that you can use.
Almost certainly greater than 30GB - but unclear if perhaps up to 137GB
(and possibly beyond), or if only up to 60GB (and no further beyond). 😀

It is a Deskpro 2000 5200MMX, using the bios that came with it (witch happens to be the newest one available). AFAIK is has trouble with 32GB+ drives.

OK so, Compaq 2000 5200MMX
well, despite Compaq stating;
ftp://ftp.compaq.com/pub/softpaq/sp16001-16500/sp16085.txt
"Adds large hard drive support (>30GB by supporting the keyword Giga) in the disk.tm"
I've never seen any evidence that the Compaq bioses of that and similar models were
able to access the full capacity of drives greater than 30GB. (In fact, my personal view
is that the compaq person who wrote the above line probably meant it to mean 'add hard
drive support up to 30GB' .)
Nobody ever seems to have modified the bioses of this Compaq or similar Compaq models
to allow access to full capacity of drives greater than 32GB size.

Your model would look like this then?;
http://www.3sfmedia.net/?main_page=product_in … &products_id=79
http://www.3sfmedia.net/images/computers/cpq- … 0166_LRG_02.jpg
http://www.3sfmedia.net/images/computers/cpq- … 0166_LRG_01.jpg
You can see that Compaq models like 4000 and your 2000 had the connector
for compaq 'Backplane Board', with PCI slot thereon. So, using a PCI card
such as 'Promise TX2' ATA controller would allow you to access drives greater
than 32GB (up to 128GB at least), although it is almost certain that the
compaq bios of that age would not have the ability to allow booting from any
drives connected on the PCI card.

Used to work on Deskpro 2ks and 4k's a lot in the early 2000's. Sadly I binned about 12 of them 10 years ago. Only rembered when I went back home in January. They are incredibly reliable.
I seem to remember if you delete the Compaq partition you got an error message on boot, but it worked just fine. I would just test with another HDD/SD card/whatever you have lying around.
I would be bit worried about cross flashing. Compaq do funny things in this area. Like your partition requirement, RAID controllers required Compaq server BIOS's, etc. Really reliable stuff for the time but also really non standard.