Thanks @Grzesiek2010. Now that you're deprecating the old device ID altogether, it definitely makes more sense to make the change in the PropertyManager class for back-compatiblity. I now have the install ID nicely appended as a querystring in FormListDownloader.java, after obtaining it as deviceId from the PropertyManager as before.
When the Install ID idea was first discussed, this post from @LN
suggested that a device ID should always have been appended to the download list url, and it was possibly an error that this had never been implemented in the app. It's certainly potentially useful for any custom server implementation, like mine - which is why I implemented it myself.
So I'd suggest adding it to FormListDownloader.java in the main repo, as I've done in my fork.
Thanks
Nik