In the Julia v0.7/1.0 package manager, is it possible to specify a specific branch or tag for a dependency that is only used in the extra / target section?
Not at the moment, no.
You can, however, use a custom project for what you wanted to do. See e.g. https://github.com/fredrikekre/Sandbox.jl where I am trying out using a test/Project.toml and deps/Project.toml environment instead of using [extras] and [targets].