Got your point. The browser default font selection is the fallback. That being said you, you can set your desired base font by styling the body in the designer, not a new class on the body element. I normally set my body font, then headings. Then I tune for different classes or elements. Want more that one font on the body? You could do this as custom code using a new CSS rule for body in the for example.
I think there are a lot of small things webflow does not expose, to protect users, I mean designers, from doing things that hurt the user experience. I would imagine that people would load up ten fonts and then complain about performance for example. I have no idea but after reading lots of forum posts I could see the point of restrictions.
Hey man, I just want external images without custom code!