One challenge is that the Legacy electronics themselves (including the flywheel encoder ring and tach sensor) are not marketed as an upgrade kit. You have to order specific parts, or cannibalize them from a donor locomotive. In some cases Lionel restricts part availability for warranty purposes. A few 3-rail Legacy products eschewed the optical tach sensor in favor of using back-EMF for closed-loop speed control. The results weren't as good and some customers complained.
If you broaden your considerations from Legacy to Lionel's TMCC command environment, you could use the Electric Railroad (ERR) Cruise Commander decoder, which also uses back-EMF for speed control but is well-liked. It's still a proprietary solution (Electric Railroad closed about 18 months ago and 3-railers held their breath until production was taken up by Sunset/3rd Rail.)
Not really sure what you stand to gain with this approach. If you invite friends over or take your locos to another layout, you would have to bring your command gear and hope it's inter-operable with whatever system they have. Personally if I were in 2-rail, I would use one of the established DCC systems, or make the leap to direct R/C with a system like Airwire* or RailPro. (*Not necessarily battery power, but R/C direct to the loco, should be compatible with any existing system.) My $.02.
