Well, it has been signed already by Richard Hayes, whose doxastic minimalist approach to rebirth is exactly the same as Fronsdal’s. Being a non-believer in the afterlife is not incompatible with being a stickler for scholarly integrity.
If Fronsdal should not wish to sign, it would be more likely out of reluctance to bite the hand that feeds him: three of his own books have been published by Shambhala, most recently a new Dhamapada translation. On the other hand, if he does happen to sign, I expect it will carry considerable weight.