It sounded familiar, but I had to look it up to recall what Helwingia is. I really don't know the answer but the first thing I would try would be fall nearly-hardwood cuttings. I'm guessing Heronswood must be doing it from cuttings since their current website mentions only offering a male clone of it.

Some super rare plants are super easy from cuttings. Last fall I attempted Heptacodium miconioides (courtesy of Heidi @ Dragonfly) for the first time and they all rooted in just a few weeks.
TTFN,
Ian

Thanks, Ian!
Yeah, this plant originally came from Heronswood, and is the broad leaf form, a male.
I'd sure like to get my hands on a female---the fruits are cool....

~Lisa