Short story, I made a program.Instructions and download here: https://github.com/mrverrall/go-row
Despite there being an appetite for rowing in Zwift the fact is a rowing machine is not a bicycle and a Concept2 rower won't connect directly to Zwift. The Zwift gods tease a rowing release every now and again, but it's been coming 'soon' for years now. Don't hold your breath.
But people do row in Zwift, so how do they do it? To get the data from the rowers computer, the PM5, into something Zwift recognises as a bicycle you need a device that translates between the rower and the device running Zwift.
There are solutions already available to do this. Some are expensive like the NPE CABLE (about £90 in the UK) and some are 'free' like the RowedBiker app. The downside with RowedBiker is that it needs to run on a extra device separate from the one running Zwift. If you have a compatible device lying around, great, otherwise you'll need to buy one. Meanwhile a Raspberry Pi Zero W costs just $10. Now I already had a Pi to hand and with it being springtime in 2020 I had some time at home to play with.
So after some not inconsiderable time spent working everything out it (eventually) only bloody works! You can download it over on github, it even comes in a nice little install package.
And now, I’m off for a row... on Zwift.