That's only ssyntax. a.b is rewritten as (get a b) where get is a generic function. You may be able to use set-funcallable-instance-function to get something closer to what arc supplies. Also there are a lot of edge cases with the ssyntax since you have to be within a w/ssyntax block. There is one included in def, so you can use ssyntax in function definitions, but you can't use it at the toplevel.
I wouldn't spend too much time on it. malisper truly has the right idiomatic approach with packages. I was mostly just flailing around with the little bit of Common Lisp I understand.