Incidentally I suspect this is what would normally fit on the end over that rubber O-ring and snake it's way up the rest of the firewall on the Senator: http://www.ebay.co.uk/itm/FILLER-TUBE-for-AR25-AR35-AUTO-GEARBOX-Senator-B-/360283397015
If you really want that easily accessible fill tube, maybe that could be bodged modified to fit the Omega firewall.
Hmm, its a thought. However I have now removed the existing tube from its rubber mounting seal, and, hooray, found a perfectly tight fitting rubber bung from my rubber seals/stoppers gash box. You know the type, with several concentric rings of rubber around the core. It fits perfectly, and is a tighter fit in the seal than was the tube, so I don't think it will be a problem, and as you say Aaron, the sump is not under pressure .
Thanks all for your suggestions, all workable I would think. At least with this solution, I will not be offering the box up, only to find that something interferes with the tube.