Hello Tav,
Looks very promising and it certainly is a welcome addition to the functionality in any case.
On the current design I would like to note that instead of using plus for enabling a style on an object, I would think using a check box with a V would be more logical. I would not use the same symbol twice for different actions as the plus is also used in another context.
You show two windows in the concept. Could you just drag and drop a style on an object to give it a class? That would be a welcome addition for the touchscreen user and mouse addicts like me. And vice versa to create a new style from the object. Less mindcraft is often higher productivity.
Additional concepts...
Would it be an idea to create a style sheet editor that opens a text editor on the right panel that shows the style of the selected object in the external style sheet? Similar to the XML editor, that will read the inline style setting.
Another thing that would be helpful is to be able to strip all inline styles and dump them into an external style sheet, possibly removing all the doubles and as such cleaning up the mess. After that, you could create a switch to add incremental style changes either inline or external as to enable people to experiment with it, with a fall back option or several undo levels. I can imagine that people would like to create some basic style sheet, which can be diverted from using inline styles for greater control. Once they are happy with the result they may want to build a variant style sheet or create an additional one so they have a basic style and can add overriding settings in an additional one as you often wouldn't like everything to reside in a single whopper stylesheet. It would avoid users to have to still manually cut the style sheets into little bit using a text editor or the like.
Added benefit is the ability to exchange style sheets that way amongst users and the possibility of eventually implementing web animation through them. SMIL is quite dead, but web animation seems to have a better future being supported by all browser implementors of importance, so eventually one hopeful might expect some brave effort on that topic, either from within Inkscape or from the outside with a third party tool that plays well with Inkscape with some level of integration. Pigs can fly if given enough thrust ;-)
Possible you might want to colour code what style is being used so one can easily see if there are different styles applied to objects. The colour could be generated from the classname of the style. Not sure how hat might work out with a bunch of stylesheets though, but I would expect that making selectable and displayable columns with coloured boxes at the end of the columns displaying information could be a handy solution to that. It would allow for something similar as the layers widget so you can easily work in a certain stylesheet.
Come to think of it, that might be a handy way to work with grouped objects as well in an object manager as well. Colour code them by ID, so one could easily identify them in a list of objects, select a group to work in and block other objects from being selected.
Ehm,.. let's stop at that for the moment.
Cheers,
Jelle