You're kind of going in two different directions here. :)
To keep this discussion focused on a Variax replacement/ upgrade, it wouldn't necessarily have to be that complicated. The (audio) output of a Variax is standard monophonic instrument-level analog on 1/4" cable. Anything more complicated (VDI) is just too accommodate control signals and power.
If you wanted to make a product that converted e.g. a Strat to a Variax, you could just mount the processor, controls (knobs and selector switch), and a rechargeable battery under a pickguard. The "end user" would just have to connect 2 leads to the guitar output jack, and possibly one more for a ground. Of course a hex pickup would also have to fit into the equation, which means a magnetic pickup rather than piezo. Otherwise you're also asking the buyer to replace their bridge. In this regard, Roland would have a bit of a head start.
As far as sending hexaphonic signals from the guitar to an outboard processor... Roland's already doing so with a number of their products. And they always seem so close to getting it right. What they did with serializing hex over 1/4" TRS is great... if only they'd managed to squeeze audio (mag pickups) over that same line... and if only they hadn't dumbed down the switching on the (still ugly) guitar wart. If they find just a little more bandwidth, miniaturize and/or dress up the pickup interface so it either disappears or adds value, and then introduce the next gen of GP10 or whatever... they're essentially already there. (Line 6 is probably quite aware of this, and looking skeptically at the potential profitability of a 3rd generation of Variax.)