Number of elongating RNA polymerases on a gene
Regulation of RNA polymerase II activation by histone acetylation in single living cells by Stasevich et al. provides detail on how they were able to determine the number of elongating RNA polymerase II for a given gene array. The issue I had with the paper is that they used a ~1.8Mbp tandem repeat as their target, so they really upped the probability of seeing activity. You may be able to do similar for your gene of interest, but there may not be enough signal to tease out your data.