It should work fine on 3.2.x :) Despite the release numbering being lower, Workbench from 3.2.x is a newer version and is backwards compatible with Workbench from 3.9 and 3.5.
Nice mockup :) I think something like that is more in the realm of Lister from OS4, which implements the icon drawing etc. itself, and offers features like the path gadget. Perhaps someone can backport Lister to 3.2 some time... Nice icons though :) When I get back to working on it I'll implement the user-defined toolbars.