There's an anecdote from Les in the Wheeler book relating to the re-introduction of the Les Paul in the late 60s, where he stated that Gibson was planning to phase out their solid body guitars altogether, and he convinced them otherwise - that rock players were clamoring to find old Les Pauls, and that they were the "hottest thing for rock". That might lead one to believe that the Sig semis were a Gibson-born idea, but can't say for sure. I still have never seen anything about the origins of the Signature series.
Then again, if he was so adamant about having his name pulled off the SG you'd think he'd have done the same with the Sig if he wasn't at least partly responsible for it.
Ah the glorious mysteries and contradictions!