How to limit available suppliers for specific users when entering "negotiated" requisition

Summary:

We have a client who wants to roll out Self Service Procurement to an additional 450 users and they will be processing requisitions for 200+ different suppliers.

Requisitions raised for these 200+ suppliers should be autocreated into Purchase Orders and so should be considered "negotiated".

These requisitions will generally be for very similar services, and they do not use inventory.

Unfortunately Smart Forms do not allow you to specify a a list of suppliers to limit the suppliers you can use and it is not feasible to create a single smart form for each individual supplier considering the volume of suppliers.

We could create a single smart form which we restrict to the 450 users, flag "negotiated" and leave the supplier field open, but then there is risk that they could raise requisitions for suppliers where Buyer intervention would be required.