I would talk to the tailor and the sheath maker and explain what you want. I would imagine the balance and heaviness of the knife, the weight of the pocket material and how high you placed the mini-pocket would be factors to consider. eg, you wouldn't want to place the pocket too high or the handle/knife might flop over, causing the pocket to twist up.
Wouldn't you want to carry a larger knife lower down for conealment and b/c there would be more grip to grasp, so you wouldn't have to reach as far anyway? Plus, less chance of the knife flopping.
Anyway, with slacks, a Delica or some kind of light folder sounds like the best way to go- it's light, you don't need a sheath, you don't have to worry about slicing your pocket or yourself on the draw, etc.