I have a dual-boot linux/win10 system, and have a mounted ntfs drive on linux with rw permission too (mount option in /etc/fstab
I can confirm that I have read/write/file exe on the mounted drive.
So I use flatseal to add the mounted folder in the allowed ‘file system’ list.
Now flatpak steam can read this folder and list it in the library storage. I can also click the ‘play’ button of the game installed there, but it always fails with permission related errors. It’s either ‘disk write error’ or ‘can’t execute *.exe’.