Is it possible to attach balloons to a parts list, so when you move or add parts the balloons stay on the same line?
If all your part are in a single assembly, and you turn the automatic sorting off, then the list should stay static and allow your manual edits to stay in place. New parts will be at the bottom of the list in the sequence added.
I have found it best to allow the parts lists to stay dynamic and sorted. I do everything I can to embed the information within the parts to avoid the need for this. I have needed to add two custom properties, or for shape names (files used with frames) and one for general lengths I simply call L.
I frequently delete and re-add the entire parts list to get a main iso view (w/ hidden lines) labeled quickly so I know what line item is what. Sheet metal parts I track with names rather than item numbers because I developed my methods in ST6 when it was very difficult to add automatic item numbers to sheet metal parts.
I'm curious as to why you would want to but I don't think it's possible to attach a balloon to a BOM at all. I worked at a place that placed revision "balloons" (triangles) next to BOM items so attaching there would be useful. As another said you'd need to make sure new parts are added at the bottom. You can group ballons and a BOM but that's only useful if you move the BOM.