While there are obviously good reasons not to allow arbitrarily long papers in the final proceedings, there are often some good reasons to allow a few papers a few extra pages. Some papers might really benefit from this, especially given the propensity of PC members to insist on adding material in the final version.
The SIGCOMM TPC chairs should be given an "extra-page budget" (M extra pages for the entire proceedings, above the 12*N page budget for the N accepted papers). The TPC chairs should have the authority to allocate these extra pages as they see fit. A paper should only be allocated an extra page if the authors, shepherd, and TPC chairs all agree that this is the best option. This should be an exceptional decision, not the default for most papers.
The SIG will not charge authors for extra pages, and offers by specific authors to pay for extra pages should not be accepted. General Chairs should ensure that there are no contractual impediments to implementing this policy, and should work with the TPC chairs to choose a reasonable value for M.
The Chairs' letter in the Proceedings should include a brief explanation of the decision to allocate extra pages. For example, "The PC chairs permitted X papers to publish up to Y extra pages, to accommodate requests by reviewers."