A few suggestions - although none are exactly what you're after...
Firstly, as most waterproof jackets reach down to your thigh, you could just wear your waterproof trousers just below belt height (leaving your belt accessible).
Alternatively, depending on your budget, have you considered Fjallraven Foresters (ProAdventure currently have an offer on), they are coated in beeswax & parafin which makes them 'very resistant to water & highly dirt repellent'.
Edit: I just noticed that the ProAdventure offer appears to be on 42" & 44" waist only.
Finally, having been on a few courses with Woodlore & Woodsmoke, I have noticed that the instructors frequently wear their belts over their waterproof jackets, leaving the knife etc. accessible (without the use of belt loops).
I hope this helps.