I think I understand that if, for example, we buy a house or a car, after marriage the "equity" in those is owned 50/50. If, after legal marriage to a foreigner, a Thai national woman is given a substantial amount of land, which a foreigner of course cannot own, what is the status of that land in the "marital assets"?

It depends. If the land was acquired by your wife through a will or gift, and the land was intended to be sin suan tua then it remains her property alone.

If that wasn't the intention of the benefactor then the land becomes sin somros.

As you pointed out, foreigners can not own land, so in a divorce, if the land was sin somros, your wife would have to either buy you out or sell the land and give you half the proceeds.

In that situation there are 3 ways to ensure you get nothing. 1. Backdate a letter saying the land was sin suan tua. 2. Put the land up for sale at a ridiculously high price so it never sells. 3. Sell it as cheap as possible to a family member and give you half of not a lot.