Agree, this is easy to implement and very useful.
Start and end point could be set by "double pressing" or "shift pressing" a step (using some button as shift - like the Seq-button). Best if one hand can be used. Some logic must be defined to decide if a press on one of the steps between start and end means "set new start" or "set new end".
Could be done by using the sequencer knobs "<" and ">" as shift keys. Hold "<" and press a step = new start, Hold ">" and press a step = new end. Minimal change, great value!