You bet! Section 4 in the Custom Reports User Guide covers the nuances of the where clause, which I think could help here. In the Actors table in the template, there's a $listUseCases keyword used to show the appropriate use cases for an actor. It sounds like this addition might be just what you're looking for:
$listUseCases where Phase = $$PhaseParam
What this does is compare the custom field value of Phase to the parameter ($$PhaseParam), which you will enter before generating the report. If the custom value matches, the use case is displayed.
Section 5.11 in the CRUG covers parameters in more detail, but make sure you define the parameter in the $info section of your template:
$param $$PhaseParam
Then, each time you go to generate your customized Project Spec report, the tool will ask you to enter a value for $$PhaseParam, and your generated report will still show all of your actors, but only the use cases that have a matching Phase value for PhaseParam.
Attached is a truncated example.
Jason