I think a willow feeder would suit you very well.
Basically you use a barrel, actually several barrels, and the poop is stored in it for two years so that it is pathogen free and can then be put safely on the ground.
This video might explain it a bit better.
If you use a barrel the right size for you to be able to physically transport it, when it's full you can just take it to the farm you mentioned and stash it out of the way until it's ready to empty. You'll end up with several barrels, so make sure they're labelled with the date they can be safely emptied.
Here's my thread on our experiments making and using one -
Experimenting with the willow feeder idea...