Implementation:
eg: 

• If obtaining the data and performing the transfer is efficient, then a normal command result can be used:

_missionList = serverMissions;

• Or, (not ideal, but) if obtaining the file list might not be instant, so you could specify a variable name to save the results in, which may not be instant. eg: it may take 1 seconds to obtain and send the data, similar to a public variable.

eg:

serverMissions "var_name";
waitUntil {!isNil "var_name"};

 
Data syntax:
The suggested array format would be a 2 string record per mission, the mission name and pbo path:

[
  ["[M-30] Operation Dangerous v3", "sub-folder\mission_name.pbo"],
  ["this is the mission briefing name", "this is the pbo file name and folder"],
  ...
]

Ideally, it would be good to be able to access other properties in every mission.pbo description.ext file, such as: author, class Header, gameType, class Params, etc, (but not absolutely essential, if it meant not getting a command to get the mission list at least).

This would extend the above array format into a multi-item record instead. This could also be a separate script command in itself, like serverMissionProperties.
 
Foreseeable Problems:

• The data array sent, might be quite large, but it would be up to the mod dev to manage any such potential issue, eg: such as only using it at the end of the game.