Interesting idea but not realistic. Historically, armies were able to survive off the land by raiding farms and such for food. Resupply really only was an issue for armament & equipment and if an army walked into a territory unopposed, it used no armament and repairs were usually minimal, although gas could be an issue.
Fleets are even tougher as no one owns a sea zone and in theory they could be resupply ed through any coastal region connected to any SC, not just coastal SCs so even if a fleet were trapped and couldn't get to a coastal SC it might still be supplied with food. I would say only armament has to be from a coastal SC as landlocked SCs aren't likely to produce armament for big guns, only shells for small arms and mortars.
My suggestion would he that immediately an ay or fleet has to have a direct route to an appropriate SC when the conflict completes at least one turn or it cannot fight. If ian army doesn't have a line to any SC for two turns in a row l, it disbands as it can live with decreasing morale for a year raiding the farms. If a fleet doesn't have access to any SC even by land or isn't on a coast for a turn, it starves and disbands. On a coast, it can raid farms the sender as an ay, so two years.
Also, if not reared after a conflict, a unit defends at half strength.