@DerekG - thanks
Easy - so simple arcs, and tell the board fab - that's it!
(good point about the scoring - it's irrelevant unless needed for some other reason)
I expected there may be some 'protocol' to specify arbitrary routing, but your advice suits me perfectly ! thanks again.
Yes, 3 simple arcs leaving say 0.6mm between them to use as snap outs (if you are using 0.8mm substrate instead of 1.6mm, then increase the spacing between the arcs to 1.0mm to ensure the round pcbs don't break out accidently during handling).
No need for any V-grooving if you only want the round circular pcbs. This will save lots of panel space which may mean you get more pcbs per panel.
For an ordinary pcb, there will often be no reason to supply a mechanical layer gerber to the manufacturer. As soon as they receive one, they will be saying "what is this for." You will supply a short explanation as to what you want them to do with it & they will then understand fully. Perhaps just confirm first what diameter routing bit they prefer to use (this is important to know if you are laying out the entire panel). On one hand a small diameter routing bit will save board space & produce less dust. On the other hand, there is considerable sideways force on it (they will probably stack up a number of panels to drill & route at the one time) & they must slow down the routing operation (with small diameter routing bits) to ensure they do not break.
If you are laying out the entire panel, ask the panel shop if they have any objections to you overlapping the circular arcs between each circular pcb. This will minimise wasted panel space. Once you have their answer to this & their answer to their preferred routing bit diameter, you will be in a position to layout the entire panel with multiple pcbs.
If you are using a contractor to load your SMD boards, just check with them if they want any fiducial marks & mounting holes (& their positions & diameters) on your panels.
Also check the maximum panel size their SMD robot & infrared reflow oven will accept.