Hmmm. At the time of adding the current simple numbering system I knew this request to do it properly would appear sooner or later. 
I think just another field in the Movements form would be sufficient (as you are suggesting) instead of a whole new 'Run Sheets' form with movements linked to a run sheet record.
However this is still a substantial change because settings need to be added for generating run sheet numbers and the new movements field needs to be handled when keying, scanning, exporting, importing, matching, etc.
I'll put it on the list.