AFAIK there’s no out-of-the-box solution for that task in MODx. However, what you demand is achievable with standard means.
1) Font size - if only you don’t define it in pixels (which I assume you are aware of when asking about accessibility) the user is capable to change the text size from within the browser. It’s up to you, the designer, whether it will result in page scaling or not (ie whether you base your layout on ems or not).
However, there are browser which don’t supply any direct way to change font size (Opera being the example) but instead magnify the whole page. If you want to provide the explicit way to allow the font size scaling you need to use a few lines of JS.
2) Template switching - if you use source ordered templates for your documents than all you have to do is to provide diffrent presentation layer, while the source stays intact. Modern browsers support alternate stylesheets, which are a great and simple way to provide diffrent styling/layout. Just use an appropriate [tt]rel[/tt] attribute in the link to the alternative stylesheet.
<link href="path/to/stylesheet.css" rel="alternative stylesheet" type="text/css" media="screen, projection" title="Stylesheet title"/>
Don’t forget to provide [tt]title[/tt] for your default stylesheet to.
And just as with font size, there are browsers, which don’t support alternative stylesheets (IE<7 for example). You have to provide JS for their users.
You might want to check this resource:
http://css-discuss.incutio.com/?page=StyleSwitching