Keynote: change the font of an equation
There’s no way to change the default font output of the standard LaTeX editor in Keynote.
There might be a way through changing the editor itself, though of course strictly speaking it wouldn’t be Keynote anymore, more of a customized variant.
So practically, no.
Old Answer (for temporary changes):
Yes
\mathfrak{x=2y} clearly changes the font in Keynote, and Pages as mentioned in the question you linked.