They could permanently mark the board when they install the RAM on it.
Since, presumably, nobody would try to sell a board with less RAM on it than shipped, a count-down scheme instead of count-up would work.
16GB board: no holes drilled in a marked area; 8GB: one hole drilled; 4GB: two holes drilled; 2GB: three holes drilled.
That would be the honest way to do it.
These things don’t matter to buyers. When the board doesn’t work right or crashes a lot they don’t care which step in their personal supply chain is at fault. They demand the original manufacturer give them a new one for free, which they believe is the terms of the warranty.