Dear henk8,
Replying in specific:
-Should I still use submaximal weights with progressive loading?
Yes. In fact, plan your weights as per HST.
-I've seen some recommendations for rugby training to include some olympic lifts (power cleans etc), any need for these?
Well, these lifts require more co-ordination of the whole body for their proper execution. They can have more functional value, to a certain degree. You can substitute one exercise for several others. Power cleans are modified deadlifts perhaps, and if you did a clean and press, you'd be blasting your delts too. If time is of essence, find those olympic lifts that hit as many muscles as possible. If possible, perform these lifts non-plyometrically, since we want muscle bearing the brunt of the weight.
-Any speed work in the gym? Or should I concentrate more on speed work on the field?
Speed work - keep it sport specific. Don't be tempted to do hopping squats with a barbell and all such risky plyometric propositions. Cheating a little on the positive to execute a good negative rep is fine. In this case, if speed work is about grabbing a ball and running for your life to make a touchdown, practice it as such - this would be out of the gym and on the field. If speed work is meant to be reflex training, again, make this sport specific - practice throwing, catching, running and intercepting balls with your team-mates.
If the athlete's body was a car, the gym is akin to the engine development room - a small muscle is a 1000cc engine while a bigger muscle is 5000cc V8 - we want to develop our 1000cc muscles into the big V8. We then decide what to do with this bigger engine - whack it into a truck (e.g. become a discus thrower) or whack it into a sportscar (e.g. a sprinter). Honing of the sport-specific skills is then akin to tuning the car after whacking in the big engine.
Godspeed, and happy HSTing