It's good to see such a great dev interested in the A500 (I have a Desire). <vbg>
EDIT: I can confirm that this works!
I cannot remount /system, either via ADB nor via a shell (on the device itself). In both cases, I get...
mount -o rw,remount /dev/block/mmcblk0p3 /system
mount: Operation not permitted
[Edit: worked with a terminal, but forgot to SU first, and I was typing mmcblkop3, not mmcblk0p3 !!]
I tried to be clever (NB: I don't have a SD card installed):
adb shell mount -rw /dev/block/mmcblk0p3 /mnt/external_sd
Usage: mount [-r] [-w] [-o options] [-t type] device directory
That's the extent of my *nix skills...