First I want to wish you all a happy New Year.
As Tim writes, must the individual selected hybrids be propagated vegetatively (spelling?), either by rhizome-cuttings, division or in-vitro. The two first ways are slow and may not be possible for all hybrids, and the third is expensive.
Until now only a very few Yuccas has been propagated in-vitro by Succulent Tissue Culture in The Netherlands and by Shady Oaks in the USA.
We don't know very well, if plants grown from F2 hybrid seeds, will grow into plants similar to their hybrid parent or not. I have noticed that some F2 seeds germinate well and the seedlings grow well, whereas others germinate bad and doesn't grow well either.
In the matter of the question about cross pollinating with other blooming Yuccas. Here in Denmark it's not a problem, while there are no yuccamoths to move the pollen around (besides the rare "Tegitecula humanioides"
). In areas with populations of yuccamoths, the flowers can be in-closed in a nylon sock until the flower opens and after pollination the stigma hole can be protected with a small piece of 3M medicaltape, or with the nylon sock again, the latter will also prevent the bogusmoths to lay their eggs in the ovary.
P.S. For those which have been wondering were I have been lately. I have been absent of various reason, and we have had seriously problems with our emailserver (I think/hope, they are solved for now).