Can I ask your opinion about using a strobe light to check the accuracy of timing on a mag ? At least one manufacturer's website does not recomend it but they do not explain why.
There is no reason whatever not to use a timing light with a magneto, although there are two significant practical issues that get in the way: 1) an appropriate, accurately located, timing mark, and 2) adjusting the timing.
1) Later non-magneto bikes had timing marks on the rotor, and even later ones still provided an access plate that didn't necessitate removing the entire primary case. The problem on an earlier bike is to find a component on which to make suitable marks in a location where oil won't spew out when it is uncovered. It sounds like you've found a solution to this on your racing bike.
2) Later bikes swapped the positions of the cam and the points, rotating the former rather than the latter. This makes it much easier to provide an adjustment mechanism to move the points relative to the cam. Again, you've solved this problem with a vernier on the magneto sprocket. However, it does require removing a cover to get access to that sprocket.
I've never done this myself, but it wouldn't be too hard to design a screw mechanism on the body of the mag that would basically replace the timing cable (I realize yours is likely locked at full advance, so you don't have a cable). You would still use the vernier to get as close as you could to the proper timing. But, during a race meeting if you found with your strobe that the timing was off by a fairly small amount, tweaking the cam angle slightly with the screw mechanism (securely located as part of the body of the mag) could quickly dial it in to being spot on.