Today I met Tim from OpenCircuitDesign, who showed me one of his Makergear M2s. There was a rather interesting mechanism on it: a magnetic head changer.
Essentially, both extruders are able attach to the belt block via small magnets, but the belt block is just slightly undersized to attach both of them simultaneously. Both also have magnetic attachment points on either side of the X axis as parking positions. Through G-code, it's possible to park one nozzle and attach the other when performing a filament switch.
More information can be found here on his site. (It's even been reported on 3ders.org!)
It's an ingenious system, accomplishing what dual X-carriage systems do without the hassle and space requirement of an actual dual X carriage. It's also simple and low-maintenance. While it's a bit niche, it seems like it could solve 90% of the problems I encounter with dual extrusion every day. There's even the possibility of correcting a difference in nozzle height through G-code.
From his design, I was inspired to start a similar project for my own P3Steel. Now, I'm not entirely sure how easy it will be, and what it would involve, but I still want to attempt it because of its potential benefits.
I've done some research, and it seems there have been similar ideas for the Prusa i3 in the past. However, the one I found is works off of two switching electromagnets, and introduces a new level of complexity to the printer. Also, the extra bulk would make me lose quite a bit of space, whereas Tim's design was just about as compact as the Makergear dual extruder was, so that gives me hope that one designed for the Prusa i3 could be similarly compact, especially with a Bowden setup.
I haven't worked out the details yet. I may have to make a few changes (which I will be consulting Tim on), such as whether the magnetic block rides on the rods or just the belt, and how to position the magnets without interfering with the belts. Just look to this project hopefully coming to something in the following months.