A bit late for OP, but it seems Ghibli Park will be selling tickets directly to foreign visitors (sales start 1/10 for visits starting 3/15):
https://www.timeout.com/tokyo/news/g...sh-site-122222
https://ghibli-park.jp/en/ticket/
It appears to be a phased rollout; at first, foreigners can only buy tickets for "Ghibli's Grand Warehouse."