I saw that earlier today, and I haven't yet figured out how that will do the job. It looks like it will certainly compress the mainspring, but it also looks up-side-down for an installation tool. The bent leg on the mainspring would rest against the bottom lug, and the straight (almost) leg of the spring would be held from slipping against one of the notches on the top lug, but even if the whole thing would fit inside a boxlock and then un-tensioned, the spring would be upside down.
I have thought about making something similar with lugs in opposite direction.
Thanks for the input!