Quick post: amazingly I got a pull request on AssistDeploy. Slightly less amazingly because it was a work colleague but they fixed an issue where AssistDeploy relied on global params instead of environment params for replacing vars in the json file.
Because there's some extra functions and switches included I decided to bump up the semver to version 2, meaning that it's most likely a breaking change. But at any rate, it'll work with variables in VSTS far better now, and will be more flexible for different tools. TBH I need to go add this to PoshSSDTBuildDeploy.