As they were already mentioned upthread, I’d say it’s not OT. 
In the movie Tapeheads Sam Moore and Junior Walker play soul duo The Swanky Modes and Fishbone performs as Ranchbone.
The Seeds performing as The Warts on The Mother’s-in-Law
Colin Hay, lead singer of Men At Work (I come from a land down under…), appeared in several episodes of Scrubs. My recollection is he portrayed an unknown busker although five seconds of research threw up a Scrubs fan site which claimed he was portraying himself.
TCMF-2L
Slade lead singer Noddy Holder portrayed a teacher in the superb comedy The Grimleys. In one episode he performs a version of a Slade song (in context of the show inferring he failed at ever realising his dream of being a musician.) Fellow UK musician Alvin Stardust also appeared in the show as a barman.
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Buffalo Springfield was the house band at The Lost Dimension in this episode of Mannix…
How about the Cuban bandleader married to a redhead named Lucy (and his band), Desi Arnaz appearing as a Cuban bandleader married to a redhead named Lucy (and his band), Ricky Ricardo.
I’m not entirely confident this fits but I will mention it anyway.
In the original puppet version Thunderbirds film (made in 1966, set in 2065) called Thunderbirds Are Go, there is an extended scene involving puppet versions of Cliff Richard Junior and The Shadows. The actual Cliff Richard and The Shadows providing the voices and music. The film was pretty much a failure.
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Ah, I got a Scrubs soundtrack with an acoustic version this on it. It wasn’t this version, which I like even better.
I have to remind myself that Men at Work was an Australian group, but Colin is from Scotland, didn’t go to Australia till he was 14.
Drifting off topic here but… Most of AC/DC were born in Scotland or England but they are an Australian band.
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Ooh, sorry, I knew I should’ve read through the entire thread!
Blink-182 shows up as a naked band in Two Guys and a Girl.
And the band Citizen Dick in the movie Singles featured members of Pearl Jam.
Courtney Taylor-Taylor of the Dandy Warhols shows up for karaoke night on Veronica Mars and sings Love Hurts. Of course we got to hear him sing the theme song to the show every episode.
In the classic film “Blowup”, there’s a scene where the protagonist visits a London club where the Yardbirds are performimg. I don’t think they were ever named in that film, so they play a generic Swinging London band. What’s interesting is that this was the short-lived line-up with both Jeff Beck and Jimmy Page in the band.
The B52s turn up in The Flintstones movie as the BC52s.
Various members of Motörhead appear in 1987 film Eat The Rich gradually being added to the Ballroom band.
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In the movie Diary Of A Mad Housewife there is a party scene and if I’m remembering correctly, the band is early Alice Cooper.
I haven’t actually seen the movie, but I think Cannibal Corpse appeared on Ace Ventura, Pet Detective.
When the band appears under their real name, but is portrayed with superhuman powers, would that count as a “fictional band”?
W.A.S.P. in The Dungeonmaster
Kiss in Kiss Meets the Phantom of the Park
In The Middleman, Varsity Fanclub, a boy band, used their real name, but portrayed the Pentarchs of Clothar, a group of alien dictators who used the screams of their fans as a power source to get back home.