Great question - I was just about to ask it myself!
We're in a similar situation in that we have pure Z ewes and a lovely Z tup to go with them.
However we also have four Manx Loaghtan ewes who ran with a Shetland tup lamb last year and each gave us cracking twin lambs. Two of the Manxs are still wild and will be going for meat in the autumn, but I'd like to keep the tamer two. However, for this to be practical, they need to run with our Z-tup.
Our farmer neighbour said that after seeing how narrow and leggy the Zwartbles lambs were, he'd do it without hesitation, but I'd love to hear from more folks who've done similar, before we commit.
Yeoman - would you be able to speak to your friend and confirm how many (if any) of his Hebs needed assistance?