Verify that you DD item OBNM is enabled for row security (ours was enabled out-of-the-box)
Using the Row Security exit from security workbench, create records for the OBNM alias in the F00165 Media Objects table.
Our following example allows Public to Read Only, while giving the Buyer group full control, for the PO Line Text attachments (which use GT4311 Generic Text Data Structure).
Your requirement would dictate just the Public record, with values of YYNY, of course with the particular GTXXX (or range of values) that applies to your situation.
*BUYER
F00165 OBNM NameObject From:GT4311 Thru:GT4311 Add:Y Change:Y Delete:Y View:Y
*PUBLIC
F00165 OBNMNameObject From:GT4311 Thru: GT4311 Add:N Change:N Delete:N View:Y
For the user, the "delete" drop down will not change, will still appear to be enabled. If the user deletes, will still get the "are you sure?" confirm box. Click Yes. But go back into the attachment, and it should still be there, as the Row Security prevented the actual delete from the database.
This is relatively new for us, so I don't know if it's rock solid; but so far looks exactly like what we've been looking for. I had found an older thread on JDEList along these same lines.