Pianotuna and Stircrazy, you're giving me hope again! After reading stevenal and your input, as well as rereading the Morningstar manual again for sizing the diversion load, I think a 24V 1000 watt element will work.
The manual goes over the power rating adjustment of ratio of the voltages squared. It also points out that the adjusted lower power also lowers the current it can accept at those voltages and that elements for much higher voltages like 120V will typically require several in parallel in order to accept all of the diverted current. At least that is how I'm reading it, they don't explain it but show the de-rated current in a table and make some statements using those currents when discussing the number of elements needed for a scenario. With a 24V 1000 watt element, I think the math works out OK with just one element. It suggests using 15V as the max battery voltage to use in a 12V nominal system for adjusting power ratings.
https://www.morningstarcorp.com/wp-content/uploads/technical-doc-diversion-manual-en.pdf
