Currently, the on-behalf-of function is only available after selecting the Requestable Offering in the catalog:
If you change the technical owner, who is not entitled for the selected RO, it's still possible to submit the request. Obviously, it fails after a short time.
It would be better to be able to select the technical owner already at the catalog page and then refresh the catalog items with the entitled RO's.
Example (clould be also on the left navigation pane):