Hello,
I know what you mean, it wasn't a 100% secure position, however, if you jack up with the trolley jack at a slight angle, this gives you enough lee way to slip the axle stand under, directly below that large bolt which bolts in the crossmember that the trailing arm attaches to. I rested the car on the stand, but had my trolley jack taking the majority of the weight, as I always tend to do. I also removed the rear road wheel and slipped it under the side of the car, just for that extra bit of piece of mind.
As regards rust proofing, I had a quick look underneath and touched up some areas with Kurust and black waxoyl. The whole of the rear chassis rail is full of manfacturer holes which will allow you to get the wand into there and spray liberally (as I will do later this summer). I had a look at the sills, and even though their covered with plastic, the drain holes are visible, and actually very large, around 90mm in diameter. Again the waxoyl gun can be aimed up each one to liberally fill the sills.
I think I have a photo somewhere of the car resting on it's axle stand, so you can get an approximate idea.

This was one taken before I cottoned onto the idea of aiming the jack slightly at an angle, which as said earlier, allowed me to support safely on the stand.
Cheers,
James