It seems that this particular plugin always uses the default project
workspace directory. And for things like windows batch commands sets
that to %WORKSPACE%. For example, If I've installed hudson into into:

c:\hudson\service

And then I create a job named "Job1". Under the advanced project
options I click the "Use Custom Workspace" option, and specify the
workspace as:

c:\workspaces\Job1

If I then setup a promotion, and create a windows batch command which
does the following:

echo %WORKSPACE%

It would end up outputting:

c:\hudson\service\jobs\Job1\workspace

instead of the custom workspace I specified. Likewise, if I use
templated builders, or try to use archive steps, etc... in the
promotion process, none of it is given the custom workspace (as one
would expect).