Thanks to everybody who contributed to this thread.
Inspired by you lot I bought a 5300 (with 3 Tycos) off eBay, and I've just spent a couple of hours dismantling my Mini 9 and fitting it, with no problems.
Yes, a couple of hours, basically being careful and taking things slowly, and occasionally using another machine to check the Service Manual (and the dismantling guide in the Guides section here).
I got as far as removing the palm-rest, but didn't un-hinge the screen from the base of the machine - I just removed the bezel while in-place. And yes, it's also possible to unscrew and remove the lid from the screen, and just leave the screen itself still on the hinges attached to the base, tho' I didn't go down that route.
Having removed the bezel, I started to remove the lid but while looking closely at what was there, and the current WiFi antennae, I decided there was no place behind the top of the screen to fit the third antenna. I thought about just placing it flat underneath the keyboard, as described earlier in this thread, but was a little concerend about placing a plastic antenna in a heat-path (the Mini 9 cools iteself by passing the heat up through the keyboard).
On staring at the screen for a while, I realized that there was an area at the centre of the bottom, slap bang between the speakers (ie behind the Dell badge on the bezel) which was the right size to take the Tyco antenna. But the bezel has some shielding there, across both speakers and the bit in the middle. But I could see no electrical/RF reason for shielding in the middle, so decided that that was just for ease of construction (sticking one big piece of shielding on is easier/cheaper/faster than sticking two small ones on). So I cut away the middle of the shielding, and ground down the plastic tab that locates the bottom-centre of the bezel, and stuck the Tyco in place.
Then it was just a case of carefully putting everything back together again, counting how many pieces were left over <g>, and then switching it on.
It booted up OK, found the new hardware Ok, I cancelled the Found New Hardware dialkog, and fired up the Intel Setup, and halfway through the installation the WiFi started to announce it had found networks!
So for the faint of heart, yes it can be done, it's not too bad a process, and you learn a lot about your Mini in the process.
Have a go yourself!