A complete dry run is complex, so we scaled back the objective in line with what you're suggesting.
Earlier this year we thought we would be able to deliver that feature, but our focus for the last few months has been on; Version 4 UI changes, performance, deploying Java applications, Let's Encrypt integration, and general stability.
Sorry I can't give you a concrete timeline for that feature. Follow our blog https://octopus.com/blog to stay on top of what features are coming, as anything of reasonable size/complexity has a public RFC. When we do get that feature we will absolutely talk about it on the blog.
I forgot to mention, we've done some work in the v4 UI to make filtering of variables easier and faster, you can achieve a similar outcome by using the Show Filter button to bring up the panel, and put in the combination of environments you want to see.