Your bank needs a resolution from each of your customer's customers authorizing your customer to apply the payees' endorsements and deposit the checks into an account that does not belong to the payees. Incorporated into each resolution will be a hold harmless agreement in favor of your bank. (You will supply all of the wording for the resolution.) Your employees need to find the resolution for each check each time these people make a deposit. If that sounds reasonable I have explained it badly.
Your risk is pretty simple: These people deposit one or more checks they had no right to deposit and your bank guarantees the lack of the endorsement. There are no limits on your liability.
I'm familiar with this because one of the banks where I worked did it, but that was in the 1970s...
Note Skyline's last sentence.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.