I created a couple of designs for toptube dent removal, but just need to figure out how to make that "8mm hole" in the seat tube more like a 14mm hole for the devices to get in there.
I doubt that the structure would be compromised by increasing that hole size.
If I still had a die grinder, I could probably do it with a stone ball, though some frames position the toptube lower than others relative to the seatpost opening.
I know that there are specialized tools that feature a small right-angle drive and carbide tip, for cutting (modifying) the port windows in a two-stroke engine cylinder, but they would be expensive.