I had to add a new category after all: gumstix. The embedded word has too broad a meaning.
Updating the kernel wasn’t such a big thing after all, even for a kernel n00b like me. I git cloned the vanilla tree from kernel.org and patched the generic (and one bluetooth) Gumstix patches from OE: arch-config.patch, board-init.patch, header.patch, mach-types-fix.patch, modular-init-bluetooth.patch, tsc2003-config.diff, tsc2003.c, and uImage-in-own-partition.patch. Most of them applied fine, I just had to manually apply simple Makefile patches and change pxa_init_irq call to pxa27x_init_irq.
I don’t know if it really works, but at least it booted:
root@gumstix-custom-verdex:~$ uname -a
Linux gumstix-custom-verdex 2.6.25-rc8-00151-gdc41023 #2 Fri Apr 4 23:43:25 EEST 2008 armv5tel unknown