How to make a homomorphic preimage/payment hash with current lightning network / Bitcoin implementations?
with the current system, it seems impossible to do so. The most probable protocol change/update will be after activating BIP Schnorr, when we have the chance to move away from payment_hash/preimage and can go to payment_point/secret_scalar.
Once we achieved this htlc's can be exchanged by adoptor signatures as described in this tutorial.
Those would obviously be additative since going from the group order to the group element is obviously a homemorphism so that we have (r1 + r2)*G = r1*G + r2*G where ri = preimage or secret_scalar and ri*G1 = payment_point