Years ago most major manufactures of Farm Tractors had their own system to attach farm equipment. Ford had the 3 point hitch, Farmall had the International Fast hitch,
Alis Chalmers had their own that I can't name right now. The Ford 3 point hitch distributed the weight best and made implements easier to pull, so it eventually won out after it came off patent. I guess my point here is that we should try and design the best system we can think of to attach the various equipment to the fixed bike unit. Ease of putting it on and taking it off should be a very high priority as well as having the tool attached very tightly so that it does not vibrate and work loose. It should be easy to fabricate and easy to duplicate for new pieces of equipment. Minimum welding, grinding and drilling of thick metal pieces should be considered in the design. That would be the way I would approach the problem at least because I am a dummy when it comes to welding, grinding and drilling thick metal parts. lol Maybe a hook or slide arrangement on one side of the fastener plate with at least 2 bolts holding the opposite side so that it couldn't slide or work loose. The other option is of course to use a different bike for each implement, it turns out that used bikes with out wheels or tires are pretty cheap and very available every where.
. Storage of that number of bikes could get to be an issue I suppose, it depends on how many applications you have for bike powered tools.
You may want some set up permanently [ Like a water pump for Lollys new well ] and some that you can change easily [ Like a grinding wheel, grain mill, jig saw, etc. ] Make the design fit your needs and how you plan on using the pedal power.