Yes it is possible. No have no pinouts/schematics.
Wish I could find the website of the guy that used an old 286 mobo for exact same thing. In his case was easy IIRC because his board chipset was all socketed so he just removed all the chipset, ram, KB controller, rom, etc. (and maybe the crystals??)
One quirk you may run into: if your SBC does not have a on board SBC keyboard connector then it could become messy because then the ISA bus has to talk to the original kb port on the donor motherboard, which would be wired differently than a true backplane with KB AFAIK.

I cutted up a broken 286 Mainboard into a passive backplane. all isa slots were in parallel.
I built a small 5 / 12 volt power supply on the back wich i can power with an old laptop power supply.
The aluminium thing is part of the "open frame case construction" 😁

Horun wrote on 2023-09-03, 00:38:

Yes it is possible. No have no pinouts/schematics.
Wish I could find the website of the guy that used an old 286 mobo for exact same thing. In his case was easy IIRC because his board chipset was all socketed so he just removed all the chipset, ram, KB controller, rom, etc. (and maybe the crystals??)
One quirk you may run into: if your SBC does not have a on board SBC keyboard connector then it could become messy because then the ISA bus has to talk to the original kb port on the donor motherboard, which would be wired differently than a true backplane with KB AFAIK.

Fair consideration on the keyboard. Actually all my SBCs are used in Amiga 2000 so they must be complete, the one I would like to test has 2x PS/2 1xVGA 1xSERIAL on the bracket. I just need electricity.

Oh now i get it. You want a (AT or ATX) mainboard shaped backplane that provides power to the isa slots?
I think like Horun said removing the chipset(s) that interface to the ISA BUS is the way to go.
The communication from the SBC to the components of the Mainboard wouldnt work anyway, so i would remove (or at least physically disconnect) everything that is not power.
The most important part is that all the isa slots are in parallel.

Are you using regular SBCs in an Amiga 2000?

So your general idea from the first post is still the best 😀

so, wich pin i must keep???

The isa slots must be connected in parallel (wich they normally are... check if -5 and -12 v are connected to all slots, sometimes they are not connected) , and you have to keep all pins connected that have either +/- 5v , +/- 12v and ground. All other pins must be disconnected from the chipset.