Interesting, I have not tried the above suggestion of using src3 on a pickup, will give that a try.
I am wondering if you even need the through or can you just use a pickup. A pickup will record dry but the loop will be played through the fx. Monitoring with fx is available. You can have a flex track playing the pickup buffer and mangle as you wish, but it will be mangling the dry buffer. If you need the processed audio for the flex you can assign a different recorder buffer to it that has scr3 as the pickup track.
I say this because it seems like you want a flex for mangling, there's no trigs on pickups. By using a thru to a pickup, you might still need a flex for mangling. With just a pickup and flex you save a track, and you will have monitoring, looping, and mangling..