Need a way to produce artifacts that are not known in advance

For some tools, the output is not known in advance and thus we need to invoke a tool twice - one to get the list of artifacts and two for the actual command invocation.
This probably can be worked around by splitting a Rule into two - one that produces a list of files as an artefact and second that reads the list and assigns tags for files.
There should be a way to do this easier.