I was always bothered by the assumption that one broadcast would get them through ALL of Federation defense and security measures all the way to a founding member world.
The impression I got was that Spock and Pardek had it all worked out in advance: there was to be an underground railroad of sorts for the dissidents, and key people on Vulcan were already expecting the Romulan ships. They just weren't openly telling the Federation or Starfleet or even all of their own government about it.
Of course, the Romulan plot in this case may well have been to deliberately incite Spock to sound the alarm (see my earlier conspiracy theory) - in which case they only offered him the chance to send the "all is well" broadcast as part of this plot. The holotransmission they said they intended to use for the fake "all is well" might not have played any real role in their plan, save for fooling Spock into sending the alarm.