The current demand propagator for addition will mark all input bits at and right of the alive output bit as alive. But carry won't propagate beyond a bit for which both operands are zero (or one/zero in the case of subtraction) so a more accurate answer is possible given known bits.
I derived a propagator by working through truth tables and using a bit-reversed addition to make demand ripple to the right, but I'm not sure how to make a convincing argument for its correctness in the comments yet. Nevertheless, here's a minimal implementation and test to get feedback.
This would help in a situation where, for example, four bytes (<128) packed into an int are added with four others SIMD-style but only one of the four results is actually read.
Known A: 0_______0_______0_______0_______ Known B: 0_______0_______0_______0_______ AOut: 00000000001000000000000000000000 AB, current: 00000000001111111111111111111111 AB, patch: 00000000001111111000000000000000