This reminds me of an observation I made that is provable in the same
sort of way (unless I've overlooked something). A "louse" is a boundary
consisting of a single degree-2 point that does not appear anywhere
else; "2." in Glop notation. We can change a sprouts game by adding
a louse to a region. The theorem is that beyond a certain number
(depending on the region), adding two lice to a region does not affect
the game.
I'm pretty sure that the result can be strengthened to show that (in the
presence of enough lice) adding one louse to a region will change the
Sprague-Grundy value of the position by the nimber *1. This should
hold in both normal and misère play.
Dan Hoey