If you can create stored procedures in the Progress database, I think you can configure the linked server to be read-only.

There are two parameters in the sp_serveroption procedure that might accomplish this:

data access

Enables and disables a linked server for distributed query access.

rpc out

Enables RPC to the given server.

If you disable dist. queries but enable RPC, then the users would only be able to execute your SP's. I haven't tested this - it's just a idea...