The license incompatibility has kept it out of the mainline kernel for Linux, so it's not really a viable option there in many situations. Linux is definitely lacking in this department.
That's a minor concern and unrelated to the portability claim. That comes down to choice, and is not a technical consideration. I'm using it with Ubuntu 16.04 LTS and 16.10 where it works out of the box. It's most certainly portable to and from Linux and other systems; I've done it personally, and it works a treat.
Minor for some, major for others. And it is technical too, because there are maintenance ramifications for it not being in the mainline kernel, for example how quickly a security patch can be applied.
And for others a constriction on which distribution they can move to etc. It just reduces the number of situations where it can be used, even if you find yourself in one where it can.
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).
"ZFS export" on the host system, remove drives, insert drives in new server, "ZFS import" on the new server, It really is that simple.