The towel bar idea is brilliant! Even better: use supply pipes for the toilet as an accessibility grab bar.
This Old House plumbing and heating expert Richard Trethewey installs a device that will heat a pool with the heat produced by an air conditioner.
This is similar to my idea to use heat dumping from a refrigerator in a water heater, except neither of those loads are seasonal.
business goes by Ted McBee Boring & Excavating
This sink adds onto an ordinary toilet.
This is a lot more complicated than a toilet with a sink on top, but might be more acceptable in a retrofit.
Lents could use one at the farmer's market.
So how much is it?
Both sites are dead but you can buy them still!
https://www.homedepot.com/pep/SINKOLOGY-SinkPositive-GreenFlow-Toilet-Tank-Sink-in-Crisp-White-SPOSITIVE-101
The specs are contradictory, but this might fit?