Like a lot of others here, we have tried the same thing in the past with out much success.
It is always adults only (with maybe a couple teen exceptions). Not really invitation only, but everyone was told exactly what to expect (and everyone was excited for it). It usually lasts a few weeks, maybe a couple months, until people get hurt and get tired and stop showing up.
It seems the class is usually filled with 35-45 year old men who want to move like they did when they were 18. So they over do it, end up pulling something, rolling something etc, and are out for awhile. "The spirit is willing, but the flesh is weak" sort of thing. If you can keep people working hard, but staying within their limits it will go a long way.
The best we ever had was when my instructor taught a kung-fu program for awhile. It was all 20-something year old guys (plus me and a couple revolving girls). My sahbonim told them he was only taking 20 students and if anyone slacked they were gone. The class was awesome- it was disciplined, intense, a great workout, and everyone could hang. After about a year though my sahbonim decided he was trying to do too much by running a seperate kung fu program from his main program and decided his TKD was suffering for it so he dropped the program and sent the students to a full time kung fu school (most just switched to TKD though).
I don't know what the answer may be. Like someone said, everyone says they want hard core training but then it doesn't last. If you can find a young enough group you could be rocking. If not, maybe the answer is something like three weeks on, two weeks off?? I don't know, something that keeps men past their prime from hurting themselves by over doing it. But maybe you have a younger group than we have had.
Good luck! Please let us know what you end up doing and how it works out.
For those of you who wish you could have a hard core class, stop wishing. Just focus on killing yourself in every class- best power, best speed, best technique. Be the first one done with a drill with the best power and technique in the room. If you are asked to do 20 pushups, do 30 in the time it takes everyone to do 20 (with good technique of course). When it is time for a water break, run to your water, squirt some in your mouth, and line back up at attention stance ready to begin again. Push yourself to your limits with every command your instructor gives you. I don't care if it is a kiddie class, if you are doing this you will get a killer workout every time. AND your instructor will notice, and more good things will come, if you have a good teacher.
Good luck!