Not that I know of.
Your best bet would be to create a "master" directory in your web root, then create multiple Flarum instances in that folder, then configure your webserver to point to the different ones.
(What I mean basically: )
web root -> master directory -> [
FlarumInstance1,
FlarumInstance2,
FlarumInstanceX
]
Then, you would configure your web server to handle the rewrites per-folder, which SHOULD be possible. If not, then you can't do this any other way.