So, I've read down through the endless posts about manual and auto CCTs. Also read a couple about converting the OEMs to manual. Those usually involve some sort of bolt with a screwdriver slot that extends/retracts the tensioner.

My question is: why cant you just insert a small flat tip, adjust to the proper tension, and then install a longer bolt with a locknut until it just touches the slotted screw inside, preventing it from backing out? Then lock it in place with the lock nut. It doesn't matter if the spring is in there or not. Opinions?

 

I just installed a new set of new OEM tensioners after riding with the stock failed ones for about a year and a half.
They were locked in place as you described but the principal of operation is different than what you described.
When you turn the "screw" with the flat blade it actually does not move in and out since it is a hollow, internally threaded sleeve, that by rotating extends the threaded tensioner pushrod as needed.
This is caused by the coil spring that is fixed to the outer housing and the above mentioned pushrod. In theory the spring and the internal threads are supposed to keep the pushrod from going back in, but as we all know, it doesn't.
When you replace the screwdriver with a setscrew/locknut, as you suggested, and I have done, it prevents the sleeve from turning, consequently locking the tensioner rod in place.
It has worked well for me but it also does not allow extension of the pushrod to compensate for wear of the chain/sliders/sprockets.
On the front cylinder you merely need a crush washer on the lock nut to seal the bolt, while on the rear cylinder you have to wrap the bolt in the area of the lock nut with Teflon tape to prevent leakage.
I would still recommend replacement of a failed tensioner but this will get you by till you have the funds or time to replace the failed tensioner.

Kai Ju