ClipperCreek sells EV chargers with a UART interface to set the charging rate (in 25% increments), so it should be relatively straightforward to stuff a Raspberry Pi or something in there.
But that option costs an extra $184, and the UART protocol is only available under NDA, which probably makes it difficult to open source.
The J1772 spec is well documented and supports the "advertisement" of maximum charging current. You could add the capability to something like OpenEVSE. https://www.openevse.com/
At the moment, I'm pretty sure I have enough panel capacity for a plain old EVSE, but if I do need something controllable, then OpenEVSE looks really interesting. Thanks.
Here's an example of OpenEVSE tracking solar capacity, which is basically equivalent to panel load management:
