Using the "List Subversion tags (and more)" parameter type will not expose the parameter as a Groovy variable in a workflow.
The "Parameters" page shows two parameters "Repository URL" and "Tag" after a run. But the parameter name I used was SVNTAG.
Building inside a free style project seems to works: SVNTAG is defined in the environment. But the parameters page also shows "Repository URL" and "Tag", not the configured name.

Other parameter types (e.g. Extended Choice Parameter) will be shown correctly on the Parameters page and work together with the workflow plug in.