thom: thanks for the hint about module-init-utils; I've now installed the package by that name on Gentoo, but I haven't had the chance to reboot into the other kernel to try it. (Really need to look into usermode linux sometime...)

Gentoo's warnings were slightly scary - they implied that module-init-tools only worked with 2.5.x kernels, not 2.4.x, so I'd have to have two versions of insmod/modprobe etc. floating around. However, based on a bit of experimenting, the new module-init-tools appear to work fine with the older kernels. Please tell me if you know otherwise... :-)