Hey Tropical Fruit Forum --
I've heard Yunfei Chen (one of the principals of the CalMei venture) speak on M. rubra twice. Most recently, he spoke before our local CRFG chapter in March.
Several points:
Mr. Chen said that M. rubra/yangmei appears to be healthier and more drought-tolerant when grafted on M. californica roots. (This is under California conditions, obviously.) He and his partners are propagating elite Chinese cultivars in association with Suncrest Nursery in Watsonville. However, as alluded-to earlier in this thread, the plants are intended for commercial orchardists and will not be available to home growers until a few more years have passed.
Suncrest Nursery is propagating yangmei on M. californica roots. So, for those in California, forget about trying to germinate M. rubra seeds. Just get M. californica plants from a native plant nursery and wait until scionwood becomes available through the usual hobbyist networks. (This still might take a while, but that would leave time for rootstocks to become well-established and healthy.)
As to where the (small initial) CalMei crop is being grown (or was grown, the season is probably done now), Mr. Chen said the fruit was being produced in association with two Bay Area-region growers. Since he explicitly mentioned the Suncrest Nursery connection, I suspect the growers are in the Monterey Bay Area, but I could be mistaken. (I got the vague impression that the plants might be happier on the California coast than in the Central Valley ... for example, he grows his own home-garden plants under shadecloth in Fremont, and Fremont has a relatively cool/mild climate, certainly compared to valley locations.)