Ok, well at least I know I can't do it on the internal SATA.
I tried attaching the SSD to the external SATA drives, and reset the BIOS accordingly. The drive is recognised, but so far I can't get the system to boot from it. It goes as far as the very first Operating System message, then the slow-flashing cursor changes to a fast-flashing cursor in the top left corner, and sits there. I suspect Windows 7 isn't co-operating - perhaps something to do with the drivers? Although - if I boot from an SSD attached to the internal SATA and add a non-boot SSD to the eSata, Windows does recognise the eSata drive.