That's about the only way I can think of to give the brush a bit more backbone. In an ideal world the solution would be to remove the knot intact, drill the handle a bit deeper and re-insert the knot with less loft. Unfortunately, I think it would be very hard/bordering on the impossible to remove the old knot without damaging either it or the handle, and probably both.
If the handle alows, hold the handle in your palm of your hand and pinch the bristles with the tips of your fingers as the old Italain barbers used to do. P.