Andy,
I had the same trouble getting the tool to bear on some of the large diameter bits for my 6" LS so I had to scheme up an "extension" tool holder. You can see it towards the bottom of this page (if the link works right!).
http://www.tractiontalkforum.com/sho...t=39516&page=8
You mention you have tried using the back of the tool post so I assume something like this still won't help! However you could make a cranked variation of it perhaps that can go in the tool post at 90 degrees to the bed and then turn 90 degrees towards the headstock, each leg would need to be sized for the depth / diameter. Maybe not a good way to explain it but I don't have any means of doing fancy computer drawings and inserting here!
As you can get it in the gap on the faceplate On your machine I would think it worth persevering with a creative tool holder to get the tool where you need it if there is no where else you can either 'borrow' a machine for a few hours or find someone to do it for you reasonably easily. Having made up something to extend the capacity at least you have it and if any other large diameter bits come up you may find it useful for them as well. It doesn't have to be anything fancy, even a welded fabrication from some thick plate will probably do the job. Just make it as heavy / rigid as you practically can.
As an aside I used this same bar to do the OD of the LS flywheel too, I think there is a picture later in the thread.
Paul.