I'm really surprised such an attack hasn't happened already. It seems so trivial for a determined attacker to take over an opensource project (plenty of very popular projects have just a single jaded maintainer).
The malicious compiler could inject an extra timed event into the main loop for the time the attack is scheduled to begin, but only if it's >3 hours away, which simply retrieves a URL and executes whatever is received.
Detecting this by chance is highly unlikely - because to find it, someone would have to have their clock set months ahead, be running the software for many days, and be monitoring the network.
That code is probably only a few hundred bytes, so it probably won't be noticed in any disassembly, and is only executed once, so probably won't show up in debugging sessions or cpu profiling.
It just baffles me that this hasn't been done already!