I had the same question.
I assume that it is reference to the steel core saturating? i.e. unable to become further magnetized.
If it was an inductor and you were applying current to the winding, then saturation is an issue. For a dynamo, the level of magnetic flux in the core is determined by the magnets. It doesn't seem likely that the designer would use magnets so strong that the core would saturate.

I have a vague memory of someone running tests in a manner similar to what the OP is suggesting. The open circuit voltage did rise in proportion to the dynamo's rotational speed.
For the OP's application, there's no particular value to the extra open circuit voltage, since the winding inductance will still limit current to about a half amp, and most lights are designed to just draw a half amp and don't care about what the dynamo's open circuit voltage might be.

Personally... for a dynamo that can be swapped between bikes, I was always fond of the old block dynamos of yore. They attached to a bracket on the fork and could be removed by taking out a single bolt. Yeah, it's still a bottle dynamo with all of their issues, but it was convenient. No wires either, unless you wanted to wire up a tail light.
My old Jos block dynamo...



Steve in Peoria