In short - I don't know of anything. The comments html code is in the calibre database, the schema of which is specific to calibre, so it would have to be a calibre plugin.
I don't need the dc:description (which is where Comments end up) in my ebook files, so I move any imported "Comments" to a custom column long text called "Blurb". Custom long text columns can be edited from the book list (in a resizable window) - it works best if you cloak the cell with an icon, viz:
Given that window is resizable, it may be possible to provide an option to have side-by-side panels of code and preview, viz:
But keeping them in synch might be tricky if editing is allowed in both panels.
Aside: if calibre shipped with an icon on the Comments column (or if a user added one) perhaps editing it via F2 the Book list could be unblocked.
BR