@Mode
Wow I'm very impressed! Your work sounds very interesting, you should post a topic about it with photos, I am very interested in seeing your work and to know more about the hybrids you have and flower designs.
I haven't crossed with my pollen intentionally because I find it hard to get all the pollen off the seed parent I end up damaging the female part. but I do have other syriacus planted next to the variegated one.
One is labeled 'lucy' but doesn't have the double flower, however it's got that deep red color and some petal growth by the pollen. It sets seed and is next too my variegated specimen.
Then I have variegated 'Purpureus Variegatus' that the flower doesn't fully open or set seed. It gets pollen but no female part, but any attempt at using it to pollinate another flower has failed. Might be worth you trying, and to get the flower to open but keep variegation.
Then I have 'danica' a beautiful double rose of Sharon that's light pink and white with normal green leaves. It has pollen but no female part so no seed, again would make a great plant if it could.
Last I have 'sugar tip', double pink flowers with light variegated leaves. One branch did set seed but seed was sterile. I do suspect that the pollen might of fertilized another of my old rose of Sharon, perhaps creating my variegated plant as it was from that one!
I have old rose of Sharon shrubs over 30 years old, they get open pollinated but they have resisted weed killer and bloom profusely. One is pure white the other reddish like 'lucy' but plant is over 30 years old so can't be 'lucy'.
I'd like to hear more from you and I could send seeds to you come fall if you'd like. When my plants bloom I can send photos I bet you'd be able to tell me if what I have is rare or valuable
