Are there proteins that stabilize mRNA:RNA polymerase or mRNA:ribosome complex?
I have searched for information on this topic as I used to work in the area of protein biosynthesis, but without success. However one argument against there being a tendency for ribosomes to fall off mRNA is the very specific mechanism for ribosome release after the stop codon has been reached (described here).

There are other possible problems with very long mRNAs:


Possible degradation of (prokaryotic) mRNA during a long translation time. I would imagine that the mRNA would have to contain structural features that prolonged its life.
Possible misfolding and precipitation of the protein product before the whole protein is completed. I think there are probably chaperone proteins that protect hydrophobic regions or encourage folding of domains to prevent this.


As a footnote, the longest mRNA is thought to be that for the muscle protein, titin. The assembly of that on the sarcomere would not be typical of polyketides, say, but other problems encountered might be similar.