As you correctly indicated, Hibiscus coccineus will not hybridize easily with other North American native Hibiscus in Section Muenchhusia. The fortunate exception is Hibiscus laevis (syn. militaris) which is the bridge species which allowed Ernest Hemming to transfer the red color of H. coccineus into H. moscheutos, in the early 1900’s. A very good paper on this subject is:
Brittonia, Volume 23, Number 4 (1971), 425-437, DOI: 10.2307/2805708
Genetic affinities of the North American species of Hibiscus sect. Trionum
by Dwayne A. Wise and Margaret Y. Menzelhttp://www.springerlink.com/content/k585573287782t57/http://www.jstor.org/stable/10.2307/2805708
The abstract references the genetic compatibility issues. A college library should be able to obtain a free copy of the PDF through JStor. I believe I now understand how Ernest Hemming came to the decision he did, but it was am amazing insight in 1900 with none of genetic resources which are at our disposal today. I am growing
Hibiscus Annie J. Hemming (PP835) which was the last hybrid Hibiscus Ernest produced and some of the flowers
have seven petals. Ernest Hemming had been working on a hardy double as early as 1910. He was also working on a hardy yellow at the same time.
Depending on their ancestry, some of our modern hybrids will backcross to H. coccineus but H. coccineus is predisposed to self pollination so you have to be very carful. Many of the modern hybrids have unusual properties which have not been adequately researched. For example, Hibiscus Moy Gande is a super pollinator, which will fertilize the almost completely pod sterile Hibiscus Lord Baltimore with a 100% success rate. My first F1 hybrids should bloom this summer; they should be very red!