Hello @benp!
Thanks for sharing this. The documentation, details and screen shot are super useful.
Our apologies as currently there isn't an easier way to share data and variables between apps (but we are working on it!) as I have seen other users also using URL params to pass info such as IDs from one app to another.
I am intrigued by why the release version isn't being separated from other params with an & character. I would imagine the URL builder deep in our code base was set up without the use case of needed to pass params between apps in mind 
Will look into that further!
In the meantime, one alternative would be to put a variable/param in local storage and then you can access it across apps. More details about that can be found in our docs here as well.
localStorage is a flexible solution for temporarily storing data locally to the browser. All apps in your Retool organization have the same origin and share access to its localStorage.