I did some quick testing, and it looks like you're in luck. Changing the uiscale CVar to 0.53 just changes the actual UI scale to 0.64:
Code:
/console uiscale 0.5333333333333333
But this command takes it smaller than 0.64 (actually 0.5333333333333333 presumably
):
Code:
/run UIParent:SetScale(0.5333333333333333)
My understanding is that these both are doing the exact same thing, but the uiscale CVar is limited at 0.64 (maybe someone can clarify?).