Great ideas! I have a '70 Triumph motorcycle in my living room so the bicycle bathroom would not be out of place... However, I am strongly leaning to the stool idea as it should be easy to do and all I have in the garage currently is a camping tripod stool. I peeled the Surly badge/sticker off the headtube intact so I could easily apply it to the seat tube, and I have a spare seat to boot.
Google turned up a fair number of things for "bicycle furniture" but most of it is arty beyond my capabilities or motivation!