Most (all?) award tickets are endorsed "NON-END/-TRAN/-REF/UAONLY" or something similar so, if that was the case, the agent was right and the ticket could not be used on another carrier.

What, if anything, is in the endorsement box on your e-ticket receipt?

In addition, the fare rules of most award tickets including the following statement in the Refunds/Changes section of the fare rules "INVOLUNTARY REROUTING WILL BE PERMITTED ONLY VIA THE SERVICES OF UA/UAX ONLY."

What did the fare rules of your award ticket say about involuntary reroutes?

I am sure others have had their award tickets endorsed over to *A or other carriers but I don't technically think it's allowed.

To be clear, there is no rule 240. It was an FAA rule that went away during deregulation. If you want to cite something that actually exists, the current applicable rule is Rule 24 in the United Contract of Carriage and, while similar to Rule 240, it it not the same. First off, if the delay is due to Force Majeure (weather, ATC, etc) UA, in its sole discretion, may either reroute you however it sees fit or refund your money (Rule 24.D). It is not under any obligation to put you on another carrier (*A or otherwise). If its due to another Flight Irregularity under UA's control they are obligated to rebook you on their own flights to your destination (subject to availability) or, upon the passenger's request and if the fare rules allow, rebook you on other carriers (Rule 24.E). UA may have more detailed and/or lenient internal guidelines, but it's crystal clear in the CoC what they are obligated to do. In the OP's case, they only have to reroute on UA due to the fare rules of the award ticket (unless the OP's was not endorsed as noted above).