Hardcore Rush is the one server that needs the most assistance. From time to time, it is populated, but it also empties quite quickly. Not many people seem to try to get the HC Rush populated, and that has been pointed out before
Just keep doing what your doing, and people will follow. If anything, head to the shoutbox and just make a brief message like 'BC2 Hardcore Rush needs you!' or something similar. (Make sure it is polite though, that often works better )
I sit in a 5 person queue for HC rush during peak hours, but late in the night, it hovers around 25 people until we start popping shutes. It just depends on the gameplay, but I have noticed that it empties out faster on certain maps, like Atacama.