Does anyone have a rule for someone using a two handed weapon with a single hand?

I was thinking tier 2 with a prereq of Strength 50 and aptitudes of Strength and Offence.

It would let you use a two handed as a one hander with a -10 to WS and -2 to damage (minimum 0)

Well this is one of the few area that DH and DH2 never really dealt with, the other is using a one handed weapon like a Bastard Sword with two hands. For these we have added +1/2 St bonus to them.

Or the Fervious long-sabre with one hand, look at the wgt and dam profile, I could see either -10 or -2 but not both

I think your rules not bad for Great Weapons used that way you are talking.