We've helped a few customers utilize their existing FC investment alongside Proxmox. But it only makes sense when there is a significant investment in FC, not when the target is the least expensive MSA/Powervault model.
OCFS2 and GFS2 are freely available. Nothing in Proxmox directly interacts with the guts of these technologies. For PVE - the target is simply a "directory pool", so nothing stops you from installing and using them.
If the requirement is to integrate these filesystems' deployment/configuration into PVE GUI, that means PVE has to:
a) dedicate development time for building packages, installation, configuration templates, API/CLI/GUI integration
b) expand QA infrastructure significantly
c) rely on major sponsors of those systems (RedHat/IBM and Oracle) to keep up to date with Kernel development.
In the end, it's all software, and possibilities are limitless. However, you can't say the same about resources.
Blockbridge : Ultra low latency all-NVME shared storage for Proxmox - https://www.blockbridge.com/proxmox