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...
Fwiw I've successfully shared my luks-encrypted usb3 zfs-formatted disk to Windows pro on my Surface 4 via a hyper-v vm running Ubuntu and samba. It won't work for all external drives - you need to be able to set the drive as "offline" in device manager under Windows in order to pass it through to the hyper-v vm (and sadly this doesn't appear possible with the sdcard - I had hoped to install Ubuntu on the sdcard and have the option to boot from the sdcard and also boot into the same filsystem under hyper-v).
