This doesn't address your question, but I saw a fairly large hibiscus growing around Potrero Hill/Dogpatch here in the city. I think they are propagated fairly easily from cuttings. Maybe that is a faster route for you, unless you have a particular hybrid in mind.

This may not be the best time of year to plant the seeds. They love heat, and late spring might be better. They don't call it "hardy" for no reason! We have one of those and it's blooming constantly. It gets very little water and is between some boulders in decomposed granite on a steep slope.

When I feel brave enough to conquer the boulders and risky enough to hang on so I don't slip down the hill, I pick the flowers fresh, and dry the petals for tea. They are edible, you know!

I got a pack of varied seeds on Ebay, and I will try them in ground in spring.

Good luck!
Suzi