I definitely agree, would be fantastic! 
For now we don’t plan to support this out of the box (feel free to create a Feature Requests for that!), but soon you’ll be able to abort a build through our API (in beta right now) with a non “error” status, which could be used to do something like what you described, e.g. with a Script step as the first step in the workflow, where you can decide whether the build should proceed and if not what “status” should be used for the build cancel. You can find some discussion here Bitrise.io API v0.1 (Work In Progress) and it’s definitely on the “coming soon” list 