Henry,
You have identified a major problem for the development of cold hardy (Zone 3) roses.
Perhaps one cultivar that is relatively cold hardy and is pistillate fertile (it sets op hips well) to look at is ‘Morden Belle’. Although it begins to flower later than most repeat blooming cultivars, it has very good repeat bloom (it’s considered the replacement for ‘Morden Centennial’). The hips are relatively small though, and I have no experience attempting to germinate the seeds. Even if the hips only have a few seeds, I think it can be considered a good “mother” if they germinate easily.
‘Winnipeg Parks’ repeats well and has good pistillate fertility. However, as you say, the Parkland rose cultivars are generally not as cold hardy as the Explorer Rosa kordesii cultivars.
I think one thing to keep in mind is that it is very possible this kind of rose material exists in advanced selections that Agriculture Canada has developed. If it exists, it remains to be seen though if it can be eventually accessed by amateur breeders.
It seems to me that not enough work has been done by amateur breeders crossing the Explorer Rosa kordesii cultivars with each other or crossing them with Parkland cultivars to develop high quality cultivars having good repeat bloom… In the process, I’m sure there would be the development of fertile pistillate selections repeating well. My only experience to date is a hybrid of ‘Morden Centennial’ x ‘John Davis’. It has good pistillate fertility but the shrub appears to be only once blooming. I should add that it is cold hardier than ‘Morden Centennial’(almost to the tips of the canes in -35C temperatures). I’ve repeated the cross in an attempt to obtain repeat blooming progeny.
One cultivar that should be used more in an attempt to develop pistillate fertile, repeat blooming, cold hardy selections is ‘Prairie Dawn’. It has a remarkable combination of cold hardiness, good repeat bloom and high quality flowers. Yet, after nearly 50 years no cultivars have been developed from it. One major negative is its lack of floriferousness. But with the right combination of the other cultivar bred with it, this possibily could be overcome to some degree.
L83 has been mentioned. Last year I crossed it with ‘Prairie Youth’, ‘Prairie Princess’ and ‘Assiniboine’ (the latter I used as the staminate parent). I have no doubt L83 has great potential to develop cold hardy roses, having high quality flowers with good repeat bloom.