Reporting back on the trip, for those interested. PM me if you want more details. (No, I don't have a GPX file.)
We camped at Mushroom Rocks on the Friday night finding a nice, sheltered spot beneath one of the giant boulders.
Around 8am Saturday we walked up to Mt Erica, then Mt Talbot. We came to the head of a river valley at 9:45am, with a clear view of Mt Tyers.
We then followed it to the base of Mt Tyers, walking about 50-100m south of the river where the valley narrowed, on the last 1km to Mt Tyers The going was varied, but generally very slow - mostly due to very thick scrub, other times due to fallen trees and litter on the ground. You definitely got the impression that no other human had been here for a very long time.
We climbed Mt Tyers (which sports a little cairn), then Mt Mueller. Different scrub, but thick and with lots of fallen trees.
At around 7pm, after 9 hours off-trail walking and 6.5 km covered, we found a rare open area to camp, about 500m from the Pudding Basin marker on a Mt Baw Baw ski trail. We found a good water source about 100m away, through - as is par for this course - thick scrub.
The next day we pushed our way through the scrub to Pudding Basin, in an hour.
We then had a relatively leisurely walk back to the Mt Erica Carpark via the AAWT.