Transfer of Plan Sponsorship within Control Group

Companies B & C are domestic subsidiaries of foreign parent A and in a control group. B is not profitable and sponsors an under funded frozen defined benefit plan for its employees. 

Can sponsorship of this plan be transferred from B to profitable company C such that C can make (tax deductible) contributions toward funding benefits for B's employees? If they were unrelated, I think clearly the answer is no (exclusive benefit rule), but OK for a control group, yes? First thought is why would C be allowed to make contributions for another company's benefit but as part of the control group, if B goes under then C is also on the hook with PBGC for under funding, so why shouldn't C be allowed to fund. If B did fail, or was sold (asset sale) with plan staying behind, C could pick up sponsorship, right, so why not now?

Should B remain as a participating affiliated employer, since it still exists and its employees are the participants? I would think so. I also think language should be clear that all contributions from affiliated employers are available to fund benefits for participants employed by any and all participating affiliated employers.