Line him up in the backfield just behind the line. Sort of how the tight end is one step off the line. He can move up to just b4 the snap.
While that'd be legal, he has to report as eligible and/or we'd need another lineman to replace his role on the LOS.
The rules require 7 men on the LOS. The two one the ends are eligible receivers. If we break the huddle and only have 6 men on the line, I believe that's illegal formation/shift. So we'd have to 7 initially.
You could work around that to how a TE and Flanker combo can shift to a Wing and Split combo, but I don't know if that applies to interior positions or only the ends/eligible receivers.
But let's assume that's legal somehow. If you're executing a shift, both players have to be set for 1 second before the snap. That likely gives the defense the opportunity to do their own adjustment and invalidate whatever games you're playing with Jenkins.
The other option, where you just line him up as a FB, have him motion and stay of the LOS would solve that mess, but we're still back to having an ineligible receiver. If he doesn't declare, he cannot run a route. You've just made the defense's job easier, as you can completely ignore him in pass defense.