How to solve "insufficent space on the device" error on Samsung Captivate
Always the way, innit?  5 minutes after I ask, I stumble across this.  It's not super elegant but its success is in its simplicity.  Simply replace the dedicated /cache partition with a link to the internal SD card, and Bob's your uncle.  Has to be repeated on reboot, so not perfect, but way better than not installing at all!