Your idea isn't entirely original
I thought of it a fair while back, I even worked out the event system required to propagate messages, and included "portals" that could teleport information FTL, for a relay system of sorts.
The technical difficulties are actually quite minimal.
Event happens at point eX, eY
Player is at point pX, pY
Each turn, until the message reaches everyone that cares, do the following:
Calculate the square of the distance between the event origin and player, which is simply:
d = (eX - pX)^2 + (eY - pY)^2
Calculate how far the message has propigated.
p = (c * t)^2
If p > d then player recieves the message.
(The squares thing is to avoid any square roots)
If FTL relays are used, then when a relay recieves a message, it creates a new message event at the destination relay(s), so this message is now propigated from two (or more) sources.
Even with a lot of events, and a lot of players that need to recieve those events, the computations would remain quite acceptable.
The only problem is having to use such a system could easily drive a player insane, or otherwise make it feel like the game revolves around fighting the SLT communication system rather than the other players
A comprimise may be possible, friendly worlds have "stargates" that can be used to transmit messages FTL, once a message reaches any stargate, it is available at any other stargate, so if you (your flagship) is currentely at a friendly world with a stargate, you can send instant orders to any other friendly world with a stargate.
This would mean the main time that the speed of light becomes an obstacle is when you send a battle fleet into the "shroud" (defintion of shroud is anywhere which isn't within a single light-turn of a friendly stargate, you dont know exactly whats going on in the shroud), the further they go into the "shroud" the longer it takes messages from them to reach you. If they are 10 ly away, it would take 10 years for you to learn of their victory or destruction. Likewise if the war was to end, it would take 10 years for them to learn that.
Or, prehaps the mission is so critical you go with the battlefleet. The deeper you go into the shroud the less you know of the state of your empire, if your 10 years into the shroud, for all you know your empire was obliterated by a hostile force 9 years ago.
Now that would be pretty cool and I think it would be managable for the players, because orders can still be given instantly to friendly worlds. Later in the game FTL communications could become more common.
Note: Players can cheat this system. Imagine an alliance of 2 players that trust one another. Their empires are mingled together and stargate networks linked. One player goes off to war with his battlefleet the other stays home. According to the game rules, Away player shouldn't know what is going on in his empire, however Home player can simply message Away player with updates of events as they happen (messaging could be in-game, e-mail, phone, etc). It's debatable if this really matters, we could even say that Friendly Aliens give each leader a special communicator, that allows FTL commincation to any other special communicator, so that messages between players are always recieved instantly, thus players sending other "shrouded" players updates would become legal gameplay and could be done through normal game channels, rather than needing to resort to e-mail or other external communications.
This concept could be embraced by allowing shared leadership of empires, so multiple players with their flagships form a real faction. That'd make for a nifty ORPG too.