G'day Dave
I've got the genuine 7A9 tool that I can loan you, and am pretty sure I've got a spare that you can have for the cost of postage. The only drama is that I'm on the road for work for the next three weeks.
After dropping the tailshaft, you can lever the seal out with a screwdriver if you are lucky. Only lever against the gearbox outlet shaft though - do not lever against the alloy rim of the extension housing, as it will readily crack. There are some fancy angled tools you can get to help with the angle (like this:http://www.ebay.com/itm/12-Seal-Puller-Hand-Tool-Remove-Grease-Oil-Seals-WHOLESALE-Automotive-Shop-Tool-/361489920953).
If that fails, there is probably no reason (other than some oil on the ground) that you can't unbolt the extension housing, and put it on a bench, leaving the rest of the gearbox in the car. You can then use the standard GMH hubcap remover to tap the seal out from the other end - see P64 of the guide.
FB/EK Holden 3-speed Crashbox Enthusiasts GuideCheers,
Harv