Can we give U grades for research and directed study courses if students completely stop research activities?

The research work performed by graduate students employed as GSRs typically has a lot of overlap with the work they need to perform to receive a satisfactory grade in their research units.  A GSR not engaging in research work because of the strike will likely not make academic progress in their units (e.g., 295-299) during that time.  However, since we are roughly midway through the semester it would stand to reason that if performance in these units has been satisfactory so far, inactivity for a few days should not drastically affect that assessment.

Assessment and grading is a faculty prerogative.  The assessment of student performance in any course can only be based on academic criteria that are consistently applied to all students.