After a fair bit of playing around I've decided that the Mango approach of a bicycle bell and a bit of string is the least awful way to get a bell into the rotovelo.
I had a pingping bell mounted next to the cockpit so I could reach out and ping it, but it was about 15cm from my ear which was not ideal. I've been looking around for a bicycle bell that I can bolt on to the shell and bike shops don't have many of those. The pingping bell mounts using an O ring so I just twisted that under a washer, which kind of worked. And on bike to work day I got a free bell with a standard clamp and one problem - the clamp is designed for a handlebar no more than 20mm in diameter, when normal handlebars are 22mm (this is why they were free). I grabbed one just in case, since they're no use to anyone else and I might be able to use it.
The other day I had a brainwave - a doorbell is designed to do exactly what I want. The actuator goes on one side of a barrier, and the ringing noise comes out the other side. All I need is a lightweight, weatherproof doorbell. So I had a look around and most of the ones I saw are designed for a thick door, not a 3mm velomobile shell. They also tend to be heavy (because who cares on a door). I did find one that I thought might work, but it was $30 and heavy, so I put that idea on hold.
(a couple more photos)
Plus I washed it. It seems odd to own a bike that needs to be washed.