You are assuming an entirely elected AC. This has never been the case. (Those who treat it as axiomatic that electing the AC is the only way will have to live with what they get.) There could perfectly well be appointments made in the second half of the year. Some kind of substitute system is probably the best compromise anyway.
Who would appoint them? Jimbo has been steadily reducing his direct involvement with the English Wikipedia, I can't see him wanting to increase it again. The current ArbCom is fully elected - one member was elected, resigned, and reappointed, but he still selected by the community (in fact, Mackensen's term would have lasted an extra year had he not resigned - his reappointment was just an undoing of his resignation, and moving him to the empty spot).