An obstacle I can see is that smallholders keep a wide variety of sheep breeds, from commercial, through rare breeds to primitives. Restaurants might prefer to be able to specify the breed, in which case it could devolve back to individual producers to have enough of their breed to satisfy the need. It depends I suppose on the number of smallholders involved.
The restaurants would need to embrace seasonal availability.
Your scheme certainly warrants further consideration and research, and is the kind of thing we would have been interested in a few years ago when we had far more animals going for meat. We breed mainly for breeding stock, so it's only those which don't make the grade that go to the abattoir.
The RBST has or had a scheme for marketing rare breeds meat, but the criteria were way too complicated for us, and the participating butchers far too far apart.
Currently if we have more sheep for slaughter than the family can use, then we sell to friends, who form a small orderly queue

The prices asked and obtained for meat by smallholders varies greatly across the board. To sell at an advantageous cost to the restaurants, while getting a good price for the producers and a cut for yourself could be a concern.