You could remove the battery and assume it might need replacing after being idle for that length of time. The pram battery also might be discharged. The MacBook should be kept in a dry place neither hot or cold.
If it were me I would try to find a custodian (maybe a friend or relative). I would use a timer with off/on switch and deliver a brief charge every 24hrs. Maybe something like 10 to 20 minutes daily to keep it at full charge.
I am aware that the older lithium-ion design cells were advised to be kept at 60% or so charge when idle for long periods but have not seen any similar recommendations for the newer Li-ion polymer design used in MacBooks.