I created https://liquibase.jira.com/browse/CORE-1746 and https://liquibase.jira.com/browse/CORE-1747 to track the features.
The reason I split them up is because while both are useful, <sqlFile> parameters may work better. The paramter logic currently works like it does in Ant where once a parameter is set, the value cannot be changed. That has advantages, but it makes it so you can't override values in a changset or changelog without breaking that logic and introducing backwards compatibility issues.
It is a common request and there are ways to implement it, though, like adding a "override=true" flag or a scope="changelog|changeset|global" flag