From The Columbia Journalism Review's Language section
"the origin of “swashbuckler” as coming “from combining a sense of the verb swash ('to act in a blustering and bullying manner') with the noun buckler ('a small round shield held by a handle at arm's length').”
How to let Swash use light (1 ac) shields but not heavy? We can already equip any shield and a script checks if we have the appropriate feat and imposes penalties if we don't have the appropriate feat. Just make the script not impose those penalties if the base item of the shield happens to be light and the character has at least one level of swash. No reworking of the class needed. Maybe just put a note in the Wiki that swash can use a light shield without penalty, but incurs penalties for heavy or tower unless they have the appropriate feat via other class or "buying" the proficiency feat.
Note that duelist AC still won't work with a light (or any other) shield equipped. This is mainly for those early levels where another 4 ac from a light shield +3 from Mudd would be a huge help. It would also help my main (i'd lose 1 ac and about 10 levels from RCR but I'd get a feat back), but I only say that for transparency's sake.