Martin,
Thanks for that - but I think I didn't make myself really clear. The point about drilling from the underside is very valid. I think that's what I would have ended up doing. However, I'm worried that, for example, the floor might be "double-hulled" or something.
With regards to the bikes I should probably have explained a bit better. I'm already using the seat-mounting points. However, there's only 2 rows of seats meaning that the furthest back row is quite far from the absolute rear of the car. Therefore it's difficult to strap the bikes in tightly on the length-axis. And if they're not strapped so that a strap is pulling the back too, then there's a danger that, in an impact or heavy braking, they will inch forward in to the drivers/passenger seats.
Therefore I'd be most comfortable if I could ratchet down the rear wheel, or pillion pegs, of the bike to some anchoring points very near the tailgate.
The idea is that we use 4 ratchet straps per bike, so we can adjust all "corners" of the bike. Each ratchet strap pulling outwards from the centre of the bike.
I've got some neat pictures of the bikes in there... My girlfriends GSX-R600 has got a very high tail, and that's only a few centimeters from the tailgate when closed - tight squeeze...