I haven't modeled a GSHP for compliance, but my first suggestion would be to get in touch with the code official to see if they have a suggested modeling path. Most likely they won't so have a suggested modeling method and see if you can talk your way to an agreed upon method. They may also ask you to get CEC/CBECC-Com clarification.
In CBECC-Com you could probably fake it to an extent by having a very low HP on the cooling tower. That said, you probably won't get quite as cool loop temperatures during early summer when ground temperatures are still relatively cool, but dry and wet-bulb temperatures are increasing. You'll still have to use a boiler for heating and you can't have an artificially high efficiency for this, but TDV for gas consumption in Title 24 isn't generally that high so I doubt it'll be your main issue.
As far as making the adjustments in EnergyPlus, this is possible but you'll need to pursue exceptional compliance. If editing the IDF directly this is pretty straightforward. To do this with OpenStudio you'll first need to custom build of OpenStudio that CBECC-Com uses because opening it in the release versions of OpenStudio makes a few changes to the model. Moreover, you need to contact the CEC about pursuing exceptional compliance as each time I've done this I've been met with considerable pushback and they've informed me that the review period may be around 3 months long.
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