If I recall the route is now a cloverleaf all around Forrest?
Last time I was at Forrest (which admittedly was quite a few years ago), I don't recall riding much that would be completely unridable on a rigid. Sure suspension would be nicer for some sections, but it wouldn't be a disaster.
I've ridden the Buller Epic trail on a rigid 2x10 and regularly ride all of the local trails (Lysty, Westerfolds, Yarra, Buxton) on a rigid singlespeed. You Yangs Stockyards is where I draw the line on riding rigid - I've done that once and didn't really enjoy it much.
The big question is if you think you're capable of doing it over 8 hours. It can be pretty brutal on your lower back and forearms, especially if you're not used to it and riding trails that you're unfamiliar with.
A couple of tips for riding rigid over long distances....
- Mountain biking is about keep your butt off the saddle and your arms loose. 10 millionty times more important when you're riding rigid.
- Get fat tyres (if they fit), run them tubeless and low pressure. Maxxis makes some great tyres in 2.2" or 2.35". Schwalbe also does 2.35" tyres. I run tyre pressures in the low 20 psi, but it can take some experimenting to find the right balance.
- Find the right combo of grips and gloves that suits you. While it might seem like a good idea, big chunky gel gloves aren't always the right idea - I find that they mess with my ability to get a good firm grip. My preference is for ESI chunky foam grips and 661 Raji gloves, which have almost no padding.
- A nice seatpost can help. For example Niner
makes one that's designed to deal with trail buzz. Don't be stupid like me and run a Thomson Elite - stiff as a broomstick.
- If it's an option, modern hydro brakes can REALLY help later in the race(although this might be hard on an old frame). Your forearms will cop an absolute beating, which will compromise your ability to brake effectively later in the race if you have crap brakes.
And most importantly... smile.
Mountain bikers don't do that serious face that roadies like to do.