Yes, this is partially a joke suggestion. But people have requested interaction with the toilet and interior of the HUB, and also complained about the non-renewable nature of biomass as a fuel. I suggest killing two birds with one stone.
The rate shouldn't be so high as to defy belief. For balance purposes, probably not high enough to fully sustain one or both HUB biomass burners, but I leave the final rate to you. Aside from the toilet humor/requested QOL aspect, this stash of biomass serves a gameplay purpose: as an emergency supply. If you happen to neglect your biomass reserves, and power suddenly fails, you maybe have enough of the stuff to re-power a few machines long enough for you to go on a chopping spree and restock with foliage. And once I've well deforested an area, this can be a source of biomass for U-Jelly pads or making eventual growbeds for Inhaler materials. As such, it provides a reason to enter the HUB proper other than the MAM.
Extra points to you, Coffee Stain, if the rate the toilet produces biomass is increased based on number of players in-game (more players, more poop), perhaps to the point of self-sustaining the two hub burners, or if you can sit on the toilet to increase the rate of "production." Please don't include fart noises or anything more than a loading screen tip for that.