Man this guy, I'm building a couple of SIPs from my recycle bin. Not using fabric pots though. Started watching Grow Tube and Pedro keeps mentioning there and briefly in the Weed Nerd live stream. Are you doing the fabric pots on perlite/lava rock/hydrotron method?
Anyway, the dudes at Grass City suggest you remove the fabric pot, water with nutrients until its done running off, then put the pot back. Pedro uses the Earth Box, which is self contained and has no medium in the res. The uses water with EM1 (LAB will work the same) in his res and top dressed with compost. The LAB will keep the anaerobic environment nice and healthy.
I have no problem adding ferments to dead res with LAB. It won't smell, will keep it nice and clean. But with the perlite bed, I would be more hesitant. Seems like a lot of surface area for algae etc, but I could be wrong.