How can I thicken a very watery Bolognese sauce for lasagna?

Definitely extra-undercook the noodles, especially if you think simmering longer could create a saltiness issue.

Use it with dry lasagna noodles, whether labelled no-boil or not.
They will absorb water as the dish bakes.
You may need to check during the baking and add more liquid.

You could try tomato paste, add by tablespoonfuls until the texture is closer to what you need then let cook to the right texture. It will be more tomato flavored in proportion than the recipe expected. Not that I find that necessarily bad for lasagna in general to have a brightly tomato sauce.

I would just simmer it longer, maybe even raising the heat a bit, and let it reduce and thicken. Uncovered.

People have been known to use roux or other use of starch for that purpose, but I really wouldn't advocate it. Simple reduction is the obvious solution, or you could puree some thoroughly drained tomatoes to add, maybe adjusting other ingredients. If you plan to bake the lasagna for any length of time the sauce can start out somewhat more liquid than you would otherwise use.