I don't see how you can express a shorter leg like that. I was thinking of something like dabaubR where each successive leg should not be longer than the previous. The only way that seems possible now is to do every possible combination of different numbers of W steps. But you understand it all better than I do
I don't think dabaubR is quite what you're after; the u should come before the d, if you want your Rook-like piece to move a friendly one. What you have there would simply make the friendly one disappear, and then move any enemy piece it captures at the end to an earlier point in its move. The effect you're after there would be more like uafafdaibR -- the first move forward sets the landing spot for the other piece, the second sets the landing spot for the moving piece, the third picks up the other piece and transports it to the landing spot, and the backward move puts the moving piece at its landing spot.
I don't think dabaubR is quite what you're after; the u should come before the d, if you want your Rook-like piece to move a friendly one. What you have there would simply make the friendly one disappear, and then move any enemy piece it captures at the end to an earlier point in its move. The effect you're after there would be more like uafafdaibR -- the first move forward sets the landing spot for the other piece, the second sets the landing spot for the moving piece, the third picks up the other piece and transports it to the landing spot, and the backward move puts the moving piece at its landing spot.