Short answer: No it wouldn't make it easier to port.
Longer answer: The Linux subsystem in Windows 10 only deals with userspace. It doesn't support kernel modules nor changes anything about making Windows drivers. Porting ZFS to Windows is certainly possible, but it will take quite a lot of effort, and the Linux subsystem is irrelevant in that situation.
Yeah, I figured as much, but was hoping there might be something about the Linux subsystem that would be helpful in porting drivers around, beyond userspace.
A FUSE implementation of ZFS exists and works well, and adding FUSE support to the Windows 10 Linux subsystem appears to be reasonably high up on the priority list.
That doesn't get you access from Windows programs, but there are some other ways to do FUSE or FUSE-like things on Windows..
It may be fairly straightforward to port the ZFS FUSE to Windows if you use things like Dokan or WinFsp which have the FUSE interface supported fairly well - these would give full access via standard Windows tools.
Last I knew, the zfs-fuse codebase hadn't been updated since before feature flags were added to any of the OpenZFS targets, so it's not a particularly well-supported solution...
Longer answer: The Linux subsystem in Windows 10 only deals with userspace. It doesn't support kernel modules nor changes anything about making Windows drivers. Porting ZFS to Windows is certainly possible, but it will take quite a lot of effort, and the Linux subsystem is irrelevant in that situation.