Thinking about the guitar tuning machines - they're designed for two things, to be able to adjust the string tension very precisely, and to hold the tension without any kind of mechanical lock. We don't need either characteristic: if there's a spring on the other end, all we need is to get some extension of the spring, and we can use a mechanical lock. Release the lock to adjust the tension, observe the spring for the right extension, then lock it down again. It probably wouldn't even be necessary to have mechanical advantage to put 5 lbs tension on the string, though it might be nicer on your hands ;)
I can think of a couple of ways to implement that. Something that looks more like a violin tuning peg could work along with the locking bar you already have (shortened, because only one side of the string will be locked, the other will go to the spring). Or a lever that allowed you to pull the loose string until it was taut, then hold the string, and move the lever a little further with a screw to add the extra tension. Or probably ten other things, none of which require tuning machines. . .
I can think of a couple of ways to implement that. Something that looks more like a violin tuning peg could work along with the locking bar you already have (shortened, because only one side of the string will be locked, the other will go to the spring). Or a lever that allowed you to pull the loose string until it was taut, then hold the string, and move the lever a little further with a screw to add the extra tension. Or probably ten other things, none of which require tuning machines. . .
