I have a GSHP and solar thermal but I would not consider putting solar heat into the ground loop to save electricity in winter. Instead, I use the solar heat to heat my house directly. There is no energy storage involved, and hence no storage losses. I can't see how, having collected the solar energy once, it would be efficient to move the heat into the ground and then try and collect it again with my ground loop.
The suggestion that this approach allows the collection of solar energy at a lower temperature is a red herring I think. I have tried doing this to heat (de-ice) my front steps, thus using the low-grade heat that would otherwise be lost. It doesn't work well because the panels either produce nothing or can produce hot water which is usable as such. There is relatively little wasted low grade heat, and any savings from running the panels at a lower temperature would be minor compared to the likely losses involved in heating the ground loop.
The only time I can see this approach helping is if the ground loop is too short, but then it is a poor solution to that problem. It would be better to extend the ground loop.
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