Hello Pete, that’s an interesting thought, a small air-source would in theory make sense as you say but then I imagine difficult to justify the expense for summer solar water heating only? Do you have any solar PV, could you automate your GSHP to turn on when the solar PV is generating? It might not be as efficient as it could be but it would still be better than direct electric and would make use of your existing investments, you could soak up the remainder: the PV generation that the ground source cant follow perhaps with a PV diverter…
@stephen has a lot of experience with solar thermal, see his post here: PV vs Evacuated solar if you were curious