This is the mechanism that keeps the bandsaw blade straight. I'm thinking it would be tough to replicate with a jigsaw, since you already have limited length of the blade, factoring in the stroke.
But I can think of a passive way to ensure perfect vertical:
If you buy a laser pointer (you'll probably need to disassemble it, if not to mount it, to replace the switch with one that's not momentary) you could suspend it so it points right down perfectly on the tip of the blade.
When cutting curves, if the blade flexes, you'd see the blade move away from the laser pointer dot and you'd know to slow down or adjust.
Just an idea.
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