Thought I'd update this thread now that I've done the job (finally
)
There is nothing under the rivets holding the rear tie down points in so you can drill away without worry. The exception is that under the left front point, there is a brake line but it is far enough away so that it won't cause any problems unless you use a really long drill bit and are a muppet.
See the pic with red circle and green line, the green line is the brake line.
I ended up putting rivnuts in so I could screw/bolt the new tie downs in place but I may very well end up making a plate to go under the floor for the bolts to screw into as I think it will be more secure and stronger.
I got the tie downs from Bunnings for the princely sum of about $12 and they are rated to 100kg and are a bit more versatile for what I want to use them for than the originals are.
I've included some pics of the finished job plus underneath so you can see what everything looks like.