Your ideas seems principially correct. You can do it all with a SDK DLL, but you need good experience with DPlay and C++.

There's a contribution by Silbergolem (Silbergolem's Nocturn - German - not DirectPlay). I think that could be the quickest way to come to a result.

Another way is to change to the commercial edition, so you can test the multiplayer WDL code (up to 4 players). It's a good way to become familiar with the multiplayer stuff for a good prize! It can make a lot of pleasure!

PS: Multiplayer coding is much more pretentious than coding a SinglePlay game. So, testing and investigating time is very important.