What problem does that restriction solve for you if you had it working?
One option would be to set a global variable or field to the ID of the client account. Any records then created on that layout could be set up to auto-enter that specified ID--thus limiting the user to only one account ID at a time.
It will help prevent the data entry folks from entering invoices into the wrong client account, they are currently processing 700 a day. It would also help reduce the number of mouse clicks as the appropriate client will have already been selected as they move from invoice to invoice.
That solution sounds like it would work out very well, I'll give it a shot. Thanks so much!