The author of this tool -- Arkyliën (https://www.patreon.com/arkylien) -- has written a program that will download missing maps rather than showing the "MAP NOT FOUND" error when joining a server. The program is still experimental, but the first version works as advertised!
Description from ModDB:DataField42 version 1
A Battlefield 1942 tool for automatic map/mod downloads.
The tool is still in a very early stage.
For sure there are bugs and missing features. Please report them to me (you can do that here)!
Features:
Download of a map from the central database after connecting to a server instead of the message "MAP NOT FOUND". (requires that the client has the mod already)(https://media.moddb.com/images/downloads/1/238/237383/Schermopname_167.png)
(https://button.moddb.com/download/medium/237383.png) (https://www.moddb.com/games/battlefield-1942/addons/datafield42)