Is cutting down a cassock just to get a certain style of opening? I think the over all look would be really that of a slightly fancier clerical shirt and not that of a jacket. I think the shoulder cape might be a bit of overkill in anything but the most traditional settings.

The biretta seems a bit out of place in highland wear, if only because I rarely see it worn as a non ceremonial item of dress these days, despite it's roots as an academic hat. Most denominations that wear them do so generally with other vestments or choir dress both of which would tend to cover up the kilt. Although the trouble maker in me is thinking if one could be diced? Although the red pom at the top might lead some to confuse you with an Arch Deacon or Monsignor.

While writing this another thought came up, of using the simple cloth buttons common on cassocks for a montrose, to give it a plain but still fancy look. That lead me to wonder if one could be made with interchangeable buttons, metal for formal and cloth for informal wear. This might end up with something similar to the idea of cutting down the cassock.