Selection marker for a cytoplasmic vector that works at low expression levels?
Flow cytometry looking for your plasmid.

These folks labeled their plasmid with a proprietary fluorescent marker, then used flow cytometry to detect cells which had taken up the plasmid.

A novel rapid and reproducible flow cytometric method for optimization of transfection efficiency in cells


  An alternative and more direct method to using fluorescent reporter
  genes is to directly label nucleic acids with fluorescent dyes to
  track their intracellular delivery [16]... 
  Using the non-enzymatic Label IT® Tracker TM Kits, any plasmid can be
  custom labeled in a simple one-step chemical reaction before
  introduction into mammalian cells [18]. Thus, both subcellular
  localization of the labeled DNA and expression reporter transgene can
  be monitored simultaneously following introduction of the labeled
  plasmid into mammalian cells [16, 18]...
  
  Herein, we demonstrate the development of a flow-cytometric assay to
  determine transfection efficiency by labeling a reporter plasmid with
  Label IT® TrackerTM. This method does not depend on co-transfection of
  two different plasmids and simultaneously quantifies cell death,
  uptake of the labeled plasmid during transient transfection, and
  expression of the target protein.


Even if only a small fraction of your cells take up your glowy plasmid, you will sort those out from the rest with flow.