He should definately be added to the site. There are many bands/artists on here who are less progressive than Buckethead, particularly a lot of prog metal artists. Look at the criteria for being progressive!
"Long compositions, sometimes running over 20 minutes, with intricate melodies and harmonies that require repeated listening to grasp."
Buckethead has a good amount of EPICS. Just look at his new (and extremely ambitious) collection he released of 13 albums. 18 of the songs clock at over 10 minutes and one of them is over 40 minutes! They all include massive amounts of improvisation, and lots of jazz/fusion integreation and/or influence
"Progressive rock was largely a European movement, and drew most of its influences from classical music and jazz fusion"
"Lyrics that convey intricate and sometimes impenetrable narratives, covering such themes as science fiction, fantasy, history, religion, war, love, and madness."
Many of Buckethead's lyrical albums incorporate the weirdest and most impenetrable narratives found not only in prog, but anywhere. Definately cover science fiction themes (he is extremely fond of robots, monsters, space, etc.) Obviously fantasy, as many songs include battles between mythical creatures and other things of that nature. Clearly war and madness are a common lyrical/musical theme as well.
"Concept albums, in which a theme or storyline is explored throughout an entire album in a manner similar to a film or a play."
If anyone has a doubt about this listen to Bucketheadland, Giant Robot or any of Buckethead's other lyrical albums. They all tell stories, and though they may make little sense, they are stories nonetheless. Even Buckethead's most classic style prog album, Population Override is an instrumental concept album, like Camel's The Snow Goose. It continuously has an underlining theme of well... population override. There's no doubt that Buckethead has used the concept album formular quite often.
"Unusual vocal styles and use of multi-part vocal harmonies."
Half of the vocals that appear in Buckethead's music are strange and robotic often inhuman. No matter what way you look at it, the vocals on Buckethead albums are very different from those appearing in modern rock/pop.
"Prominent use of electronic instrumentation"
Buckethead often incorporates synthesizers into his music. Population Override is a prime example of this with it's extended synth solos, very similar to the classic prog style we all know and love.
"Use of unusual time signatures, scales, or tunings"
I am not sure how much time signature changing Buckethead uses, but he's used just about every odd scale one can think of. He's used scales ranging everywhere from standard to middle eastern style being able to touch on almost every style of music, making him more versatile than most prog musicians. If prog isn't about constantly changing and evolving then I guess I don't know anything.
Honestly the least we could do is add him to Prog Related. No one can say artists like The Doors and Blue Oyster Cult are more varied and progressive than Buckethead. One may not enjoy his music, and may find him a joke, but the truth is at one point or another he has touched the genre of prog.