There was an unofficial hardtail cat in the past, (last year was very kindly sponsored by Cy - £50 for the winner of each round and a Bfe frame for the overall winner. Unfortunately, Cy couldn't make the rounds and none of the organisers were particularly interested in keeping tabs on the hardtails at the rounds I was at (with the exception of Eastridge last year, when Dan Stanton stepped up with a pen and paper and sorted out a podium), so it was always down to the riders to sort out. There's not been one this year.
I did Inners, Dyfi and Eastridge last year, and Afan, Hamsterley (all on a Soul) and Dyfi (on a Slackline) this year. All brilliant fun and anywhere between 5-15 hardtails at each round, everything from 4x bikes to XC 29ers. Didn't get an entry for this weeked - and I'm gutted about it!
No specific hardtail tips really... You're always going to be slower over the rough bits, so I just try and be smooth there and then really get on the pedals on the gentler stuff and make up time. You seem to get battered a bit more through the day too.
Overtaking - I've generally had no problems overtaking, although I know others have. If you have an ok seeding run, you'll have few problems and you'll probably not be passed either (unless the guy behind you crashed on his seeding...). If you seed badly, you'll have to pass people. The only time anyone's not let me by, he crashed after about a minute or so anyway.
It's basically just a shed load of fun no matter what bike you're on.