Rightsizer cannot access Safari's own zoom function, nor can any other extension. However, there is a way you can do essentially the same thing as Safari's built-in zoom in Rightsizer.
First, you have to select the Rightsizer option "Use CSS zoom instead of font size". Then, to zoom a whole web page, type Command-A to select the whole page, and then do the click+A thing as usual.
The main drawback to this trick is, of course, having to enable the CSS zoom option, which means you can't use the font-size method on specific text elements. I will think about adding a special key combination to do the whole-page zoom thing more conveniently.
Chul