i would think they would have a nice martial property for that weapon. smiting and disruption for example, though if we want to be more creative we can look at other properties.
Instead of immunity to positive energy, which would prevent healing spells from benefiting it, how about we borrow the following?
Positive Adaptation (Ex): Vivacious creatures do not exceed their full normal hit points or need to make Fortitude saves due to being in a positive-dominant environment.
i never thought it would happen, but we're actually whittling this list down nicely. current count is about 70, but when you consider how big that list was initially, and how many revisions we have done (and how many were knocked off that list by WotC and Paizo doing their own conversions), it is pretty cool how far we have come.