First off, these look plenty Orky. Nice work. Many have spoken about using the Fantasy Black Orks in 40K, but this is the first I've seen where it has actually been done. BTW, these would make fine 'Ard Boyz as well when they are required. They definitely have Eavy Armour, but not as heavy as what is worn by the Mega-Nobs.
I also think using an existing list (any) and proxying Ork Models is brilliant. Of course they might cringe at tournaments, but I think any friendly game would allow it. I would. It is too cool not to do and all the rules are already laid down.
OIC, I am impressed with your creative konverting skill. It is nice to see this coming out.