From what I've seen, I don't believe this is possible. I was in a similar situation where I setup a GRUB entry to boot from a USB. If I didn't have a USB stick in before startup, the boot option fails. If I boot with the USB stick already connected, the option would boot from the USB.
Using GRUB 0.97, I've scanned the manual and tried many commands but I didn't see anything related to re-initializing the disks.
I see many hints in the manual that GRUB relies on the BIOS (if not entirely). When playing with boot options from the BIOS, it's in the same situation where if I don't have the USB stick plugged in before BIOS starts, it will never be listed as a bootable option (using AMI, Aptio). Maybe this is a BIOS limitation, a limitation of both or I could just strayed on a tangent.
I thought this was going to be an issue for me, but then I wondered what was easier: having someone toggle to the GRUB command mode and reinitialize disks or just CTRL-ALT-DEL'ing...