west,
In that case I believe you can do it as follows for each letter with one or more counters, Smart Guides being your friends:
1) With everything deselected, hold Alt/Option (to select the whole path) and click with the Direct Selection Tool on the border of the counter (the inner path in the compound path) at a spot where Smart Guides say path (not anchor), then for the other counter(s), if any, repeat also holding Shift to keep both/all selected;
2) Hold Ctrl/Cmd and click C then F then X then F to have a copy of the path (set) at the top of the stacking order in the expanded Layer in the Layers panel, then switch from Fill to Stroke in the Toolbox, then set the Stroke Weight to 0.001 in in the Stroke panel and change the stroke colour if needed, then hold Ctrl/Cmd and press G to Group if there are more than one counter.
If you wish/need to Group the cut line and the letter:
3) ClickDrag across the whole letter and Group.
Below is a B made as described at 72pt with the font interior vector line set at 0.001in shown in red, scaled up to make the line clearly visible:

Click/RightClick to get closer, Click again to get closer still