I would just totally remove enough bricks fom the area large enought to complete the window work, then hire a mason to reset the bricks. Just remove the two or three courses that will be above the window, because they are going to be in the way of installing the lintel anyway. The mason will weave the bricks back in and return the end pieces properly for a quality finished job. Just cutting through the bricks as you suggest will not work because most residential bricks are cored (have holes through them) and cutting is going to expose them to view.The mason will also set the lintel as he lays up the bricks.Remove the first couple of bricks by either cutting the morter joints, or just beat them out with a hammer and chisel.Once you have a couple removed you can salvage the rest by tapping along the morter lines with a brick chisel until they loosen. Just take your time and you will be able to save enough for the repair work. If you get a mason to look at it first, he may recognize it as a readily available brick and not woth the trouble of trying to save.
And like Ron said, you will probably need to install a header directly under the wall plates anyway.