Can people chill cross league in Guild Hideouts?
Of course you wont be able to trade or join party, but just be in hideout.
They are not cross-league. Each league has its own guild hideout shards.
https://www.reddit.com/r/pathofexile/comments/q87gsj/-/hgswl5h/?context=3
It's worth adding that they can't be accessed by Solo Self-Found players either.
Edit: They're going to attempt to support this so it may be do-able after all.