Well, the uSD card itself works fine. I took my spare-spare N900, also with U-boot (this time latest version, but shouldn't matter), and Nemo booted just fine.

Actually because of this major fvck-up I had the opportunity to spend a couple of hours with Nemo. Nice experience.

So, uSD card works fine. U-boot should be able to boot off it.
The only small-ish problem is that the affected N900 doesn't seem to want to cooperate with either microUSB or microSD.

Does anybody have experience with using the debug ports under the battery to create a USB connection that can be used for flashing?

I *will* search for relevant information here, but any pointers will be much appreciated