Old post, but in case it's useful in the future, also a third option is to follow someone whose instance has communities and boosts at the same time, and follow him/her. Whenever the post is boosted, it gets propagated to the Mastodon instance of the follower.

Mbin does this, PieFed early on said they wanted to implement a similar hybrid system as Kbin/Mbin but dunno how it's going, and with how overarching Misskey seems to be for text/image posts, I wouldn't be surprised if one there could follow communities too.