I've just tried this to allow branch specific versioning of a package that I make regular small changes to (meaning updating a tag seemed excessive). I tried creating a branch named 1.1 (based on the most recent tag being 1.1.12) and then just commit small changes to this. Unfortunately Composer must use a test for if the branch name is numeric and try resolving it to a tag - I got the usual (and totally useless) "Requested package could not be found" message using "dev-1.1" but after renaming the branch to "one-point-one" and the requirement to "dev-one-point-one" it works. A way round this would be useful if one existed, but it works for now.