I don’t know the direct answer to your question. But I was at one point trying to do this same thing, and eventually decided not to use fixation for this purpose.
Instead, I keep an extra Ruminate module around which does nothing except play the loop normally. Its CURRENT output then serves as a phasor that is always perfectly synced to the main loop. You can then manipulate that using HetrickCVs phasor modules, patch the manipulated phasor to the SET input on a different Ruminate (“fade on move” must be turned off), and patch that to the audio out. Example here: Stochastic Telegraph Announcements - #72 by andreya.ek.frisk
There may well be a way to do this with Fixation too, maybe Mahlen knows. But in case you find this other approach useful, well there it is. 