I think western riders, at least good ones, want the horse on the bit, too. Just that since a curb bit is so very much more powerful, the horse will maintain more of a 'cushin" of distance off the bit itself. But he is still connected to the bit and influenced by a bit that says, "no further than here, please". The good western horse respects the limit set by the bit and is then asked to work up inot the bit, just as a dressage hrose.
The dressage horse may have a more "intimate" feel on the bit and rein becuase he comes right to the bit, no cusion of space. But, he should not be leaning on the bit any more than a western horse is allowed to lean on the bit. He comes to the bit, but contains the push behind it , by having more lift through his back and shoulders.
You can work on some topline improvements without seriously "pushing him into the bit". the long and low exercizes would have really minimal push. It's more of a "let" than a push, with some more encouragement for the horse to push under himself. For this, it does help to have a connetion to the bit, so that you can tell the horse "not faster, deeper" I think you can do this in a western way with out compromising your training.
Doing things like hill work and over cavalettie also help with topline. Topline comes from abdominal work, which comes from having to lift the legs higher and more under the centerline of the horse.