I'm not sure what you mean by "heat pipes", but you want to go with an enclosed collector of some kind. I agree with Jeff that drainback is the way to go. No antifreeze to deal with and very reliable, keep it stupid simple. I've built a drainback solar preheater in a similar setup so hopefully I can be of some use.
A few things to keep in mind with a drainback: Make sure the pump has enough pressure to push the water the entire height of the pipe. Have a space between the water level of the tank and the bottom of the collector return pipe, you need a siphon break for proper drainback. Snake the pipe inside the collector with a downward slope, I've had good luck with pex-al-pex pipe as flexible, durable, and less expensive option.
A few other suggestions: Place the preheater as close to the water heater tank as possible, so you don't have as much "lag" before the preheated water reaches the heater. Twinwall polycarbonate is the way to go as far as glazing is concerned, the single wall I've used, while cheap, leaves more to be desired in terms of aesthetics and efficiency. If possible, base your collector box dimensions off of your glazing material dimensions. A little planning (as I've learned) would have reduced glazing scrap waste and resulted in a better looking finished product.
Depending on how you size your preheater, you might want to consider downsizing your water heater. If you are only heating water say 20f-30f from 90-100f to 120f, versus a 75f rise (55f to 120f) in a regular configuration, does it make sense to keep all that unnecessary heated mass sitting there 90% of the time slowly leaking thermal energy? You just want to double check the recovery time and be sure it works for your usage. Then again you have a sweet heatpump
, so I wouldn't mess with it till its time to replace it.