I recently bought a Mustang with a through-body system, and I'm trying to figure out the right strings for this. Despite how long I've been playing, I'm only a recent convert to flatwounds, which I love on my Fender P. I've learned Labella makes some specifically for these Mustangs, so I've ordered a set. But I also have a set of the T-I JF324. According to some accounts I've read, these will work on a Mustang—but based on my measurements, the silk will not entirely wrap around the post on the low E. My understanding is that with flatwounds (or is that just Labella?) this will break the string. I don't want to find out the hard way, because these are expensive, and I can still return them. Having played some type of Fender P type bass my entire life, I've never had to struggle with string length considerations before—this is all a bit new to me. Any suggestions on roundwounds for the Mustang are also appreciated.
It's true La Bella does explicitly warns against having any of the main body of the string wrapping around the tuner post. That said, TI Flats are a lot more flexible and pliable than the La Bella flats, so you should be okay with the E string being a bit too long on your bass, especially with the big fat Fender-style tuner post.
TI's are such rubber bands, they should work body through, and may not be an issue on the E tuner post if you go slow. After all, I have Fender 9050CL's body-through on my custom P-style fanned fret bass, and they work great, granted they are designed with the shorter wrap length for the E string so only the silk goes around the tuner post. The reason I mention Fender is that on one model of the Mustang, the Fender 9050L's (100 E string) are stock, and the speaking length does wrap around the E string tuner post.
I have an American Performer Mustang set up with La Bella 760-MUS flat wounds. The "MUS" stands for Mustang. I"m very happy with these strings.