In the BEopt 3.0 beta, the inputs for HPWHs have been significantly simplified so that you no longer need to provide detailed inputs (compressor capacity, COP, SHR, airflow rate, etc.) that are nearly impossible to find. The model just needs UEF as an input. So you should find it pretty simple to create a user option in BEopt v3, whereas yes, it would be very difficult to create a user option in BEopt 2.8.

As an aside: Note that while there is a penalty for using a larger tank size for a conventional storage tank water heater (because you have increased tank surface area, and therefore increased tank standby losses), there is generally a net benefit from choosing a larger HPWH. This is because the HP has a much lower heating capacity, so you are more likely to use backup electric resistance when the tank is low on hot water remaining. If you have a larger HPWH, then you are going to be low on hot water remaining much less often. So being able to use the HP more often compared to electric backup often provides increased performance that more than offsets the additional tank losses.