I've been in correspondence with Alan Tharratt, another narrowboat user with an Envirolet composting toilet aboard. He fitted it himself, and was kind enough to share his thoughts with me:
We have had an Envirolet composting toilet for almost two years and are very happy with it. They are certainly less hassle than any other sort of boat toilet.
We don't live on our boat but we do use it for several weeks and lots of weekends each year. We haven't had to empty it yet although we may need to fairly soon.
They are quite large and would be difficult to retrofit in many boats. The fan runs all the time and can be a bit irritating sometimes.
I bought the toilet direct from Envirolet in Canada; much cheaper than buying in this country. I fitted it myself, very simple, just cut a hole in the roof and provide power.
The fan has two settings; it is turned down at night but it's still a bit noisy, though to be fair this is partly my fault. It should have a wind turbine thing on top of the flue outside, it's quieter with this on, for some reason.
But I misjudged the height of a tunnel last summer and knocked the thing off into the canal. Not got round to replacing it yet.