What approach can be taken to create a sugar free Butterfingers clone?
I think what you'd need is actually a sugar-free hard candy recipe - something like isomalt, or erythritol, with additional sweetner.  The crunch in butterfingers candy is actually thin thin sheets of sugar crystals, folded around, separated by and layered with peanut butter.   There are a number of sweets that use the same basic principle, using fats keep layers of sugar separate in fragile layers instead of staying in chunks or dispersing smoothly.

For a basic technique, you would need the hard candy recipe in a half-molten, taffy-like consistency, it needs to be soft enough to fold but sturdy enough to keep together.  When you add the peanut butter, you would use a flat spatula to mix, so that the candy would stretch and tend to lay in flat layers as it's being folded.  

You would need to fold a lot - the more layers, the thinner each layer is (think how thin the layers in a butterfingers is, thousands and thousands), and although it is pretty messy at first, more layers means more surface area for the peanut butter to cling to instead of sloshing around in puddles, until they're not separating much if at all.  Once it's pretty well folded, the mix can be spread out into a pan and let cool.  The more carefully you fold, the more consistent and even the layers will be - but if you just want some crunch, a fairly sloppy technique should still get the job done.

You can warm the ingredients if it cools too much while mixing, you just have to be careful not to overheat, lest it turn into a smooth mixture instead of staying in layers.  Obviously the warmer the mix is the easier it will be to manipulate and fold, and the cooler the more likely to stay in layers instead of intermix - that's why the taffy stage is best.

Of course, this is a very basic explanation of just the technique - you would do better to find a sugar-free hard candy, and substitute it into a proper butterfingers clone recipe for things like amounts and ratios, not to mention supplementary ingredients.