Yes, I have the Fralin single-coil pickups in my M5V. It's been quite a while since I had the OEM Duncan pups in it. I hate to admit it, but I don't really remember how the Duncans sounded now.
I went from the Duncans to Nordstrand NJ5S single coils. Rocked along with those for a couple of years, but still wanted a little different. A tad more scooped, a little sweeter highs.
The thing with the Lull pickups, as explained to me by Lindy himself, is that the Duncan 5-string pups are a little shorter than the Fender-sized 5-string pups that are made for the M5Vs. Fralin doesn't have a pickup form in in that size, but he was able to take my Nordstrand pups (which DO offer that size) and rewind them to his specs.
I dropped the newly-wound Fralins into my M5V and I was smiling within 30 seconds. They sounded positively wonderful with the John East J Retro Deluxe preamp.
A little later, I was feeling the urge to move away from the baked-in warm tone of the J Retro to a more transparent, but still with character-type preamp. I went with the Trickfish IPA preamp. Once again, I'm smiling. Nice clear highs, not as heavy-sounding with the lows, but still enough fullness and body in the tone. Just a more "open" tone to my ears.
While the M5V sounds great as it comes OEM (Bart preamp, Lull/Duncan pups), which is what drew me to the M5V in the first place, I was looking for a tone that I could play go fingerstyle, then go right to a slap style if necessary without changing tone settings.
I was being kinda picky, but after 3 sets of pups and 3 preamps, I'm finally happy with the combinationIt was a little trip around the world getting there, but it was worth it. Now when o pick up the M5V, it's all me and what I like for tone, for better or worse. I get the "I didn't have to touch the EQ" from most sound guys.
(I think my Summit Audio tube DI is part of that equation, too….

).