While Mr. Pall's horticultural advice is no doubt accurate, this Hibiscus is an experiment meant to absolutely disregard Walter's own personal desire to recreate the aesthetic of natural form, while still maintaining a composition which is impressive to the public. Despite this I also like it, though it reminds me of the Emperors Clothes.
Many Rose of Sharon (Hibiscus syriacus) are perennial shrubs in North Carolina and there are likely tons of aging landscaping specimens growing outside of dusty one story houses which might be large enough to form a proportional and stunning bonsai. They bloom through most of the summer season. I am growing out one which has quite a straight(long internode) growth habit with narrow branch angles, which may be common to all of them.
Pavonia praemorsa (From Meehans Miniature) is a tropical that has small (1") hibiscus like flowers, and remains quite lush even when grown in poor conditions indoors, and may be a good alternative for bonsai despite the tropical requirements .