I reached out to some people on the #sigrok IRC chat (https://webchat.freenode.net/). They seem VERY interested in getting it working and included.
They suggested I clone the libsigrokdecode repo, add your 2016 version PD and commit changes, then do other changes I have done to try and get it working and commit them as well. Then connect the clone with a github account, provide the URL to the public repo in the IRC chat, and they can review and suggest changes.
The problem is I'm not very familiar with git because I've never really used it

They also requested I get plenty of example captures: "example captures go to the sigrok-dumps repo (another clone which you'd amend), see existing directories for README files that should accompany the .sr files"
There is also a new page on their wiki: https://sigrok.org/wiki/Protocol_decoder:J1850_vpw and they've given me a wiki account and asked me to add to it. If you ask for an account on their IRC chat they will give you one as well.