I have a Pale Mild recipe I have been wanting to do this way. Ferment in the pot until close to terminal gravity and then rack to my cask to complete the ferment and condition. If your worried about dust or dog hair or something floating in there just stretch a piece of voile fabric (BIAB bag material) over the top of the kettle and affix around the rim of the pot with a large elastic hair band. Voila, open ferment in your pot.
Don't like hops and trub in with the fermentation? Do the opposite of whats commonly done. Instead of racking to the fermentor the top 5.5 gallons of 6 gallons and leaving the hops and trub behind, instead, rack off the bottom 0.5 gallon containing most of the hops and trub.