I thought I’d wait & see whether anyone came up with some new ways to solve your use case but here’s a couple of option’s that Monzo have built & are working on -
If you’re an iOS user, you can achieve something similar to what you’ve described using the targets. Just set your category targets or just your total monthly target so that you’ll end up saving however much you want to set aside, if you don’t exceed them. This feature’s on it’s way for the Android app.
But that’s probably not exactly what you were looking for so the good news is that Monzo are planning to set up virtual pots too. We don’t know much yet, in terms of how they’ll work but since Hugo commented that they’ll be a good extension of targets, I’m imagining something along the lines of what you’ve described 
In other words, I’m hoping we’ll be able to set money aside in a pot to ensure that we don’t spend it. To pick up on @milleniummanp7’s point, I’m not sure that I’d want my balance to exclude the set aside money but it would be good if my targets counted the money as already spent / unavailable.