I think there are several in-theory possible ways how to implement "games list" functionality.
1. develop external program as mentioned above + custom master server = probably most easy solution, as it can be developed by almost anybody
2. develop client mod communicating with custom master server via our custom protocol (if possible, but i think yes and maybe markieboy was mentioning this..?)
3. modify swat 4 settings to connect to our emulated epicgames master server
4. (in theory) modify swat 4 settings to connect to our emulated gamespy master server
The nr 3 & 4 would require reverse engineering of current communication to the master servers. Communication to the gamespy server and epicgames server is totally differemt. Also, from what I learned, it may be possible, that epicgames master server does not support filtering games, etc. It does not support sending stats for sure. Therefore I think the best solution would be nr. 1 or 2 as they could be "easily" modified in future.