But not sizing specifically, right? It looks like I can use the system typeface but not my own typeface at system size.

It looks like it supports sizing:

> "Going beyond the system font, iOS has dynamic type behavior, which can provide an additional level of fit and finish to your content. These text styles identify more than simply a particular font family; instead, they represent an entire style, including size and weight. "

Note that "dynamic type" in Apple vernacular refers specifically to the variable-sizing accessibility feature, so I'm pretty sure this does what you'd want.