during the Overture on The Tubes “What Do You Want from Live?” album, right before a guitar mini-solo, Bill Spooner (I think) sings:
“…that’s how our guitar player, Roger Steen plays it-- listen to him–he plays it so TO-GETH-ER… 'cuz that’s how he likes it…”
this is kind of a fun variation on what the OP was referring to…
How about Jon Spencer right before Judah Bauer’s solo on the Blues Explosion’s “Dissect?”
“Play the blues, punk!”
I think this was also meant as a reference to their music often being described as “punk blues.”
In a similar vein, Freddy Mercury introduces each of the members of Queen in “The Invisible Man”
“John Deacon!”
“Brian May, Brian May!”
“Rrrrrroger Taylor!”
I was going to mention that. I think that’s the best variation of “Guitar!” I’ve heard. It doesn’t hurt that the solo after that is great as well.
The whole song There’ll be some changes made as performed by the late Chet Atkins and Mark Knopfler (still very much alive) is chock full of this sort of thing. Hilarious stuff. 
Wow! Monkeypants steps off the plane from Prague and his red-headed stepchild of a post has become a 44-reply behemoth! Groo-vy!
So the thread has multiple personality disorder – that’s ok. To sum it up, we have:
Now, my OP was meant only to deal with #1, but incidents of #2 do embody some of the negative elements of #1. Thanks everyone for unexpected and fun reading. And I stand by my words: Heart were formed when a baboon’s ass sweat dripped into a termite mound.
Now, bonus points to whomever can identify the Paula Abdul song in which she shouts, “Guitar!” before the solo.
Disclaimer: bonus points cannot be collected by next of kin of winner if winner’s cause of death is stoning, drawing and quartering, defenestration, shooting, stabbing, human cannonballing, or dropping into a volcano, and death occurs as a direct consequence of answering question.
My response must’ve gotten lost in the crash. Jonathan Richman does this not once but twice in “Important in Your Life.”
It’s been a very, very, long time since I’ve listened to KISS, but I know Paul Stanley did it at least once.
I think it was on the live version of “I Stole Your Love”.
(maybe because Ace Frehley was so fucked up that if Paul didn’t remind him, he’d forget to do the solo?)
I remember the Dead Milkmen doing it at least once, but can’t remember the song. He says something like, “Play it Joe!” The great part about it was that it announces the worst guitar solo imaginable. 
How about as a signal to stop? There are some bootleg Strawberry Fields Forevers where you can hear John yell “That’s enough Ringo” or “Settle down Ringo” at the tail end of the song before it completely fades out.
What if during a live performance the singer yells “Guitar” right before the well rehearsed solo, but the soloist finally tires of being told what to do and refuses to play. Anyone ever see that happen?
Bradley Nowell yelled it in one of Sublime’s million different remixes of Don’t Push. He then proceeded to say “bass!” before the bass solo and “come in, Bud!” right before the drums.
For the record, while the late and lamented Michael Hutchence did indeed yell “Trumpet!” beforethe sax solo on the recorded version of “New Sensation,” on live versions he can be heard to say, after the solo has concluded, “That ain’t no trumpet.” I always get a giggle out of that. Can someone tell me again why he’s dead?
I hate in songs when the band mentions the name of their band or the date in their songs.
311 is guilty of both of these crimes.
hmmm, i dont recall heart ever doing this. maye I hav an example (if its on or after bad animals it wont cause me to dislike them as those albums already suck, but some of their earlier stuff is good)???
who can forget ween, on “powder blue” , where he introduces the entire backup country band, up to, and including, Muhammed Ali?
Well, slightly left of topic. The song Victim of Changes by Judas Priest contains the lyrics “I’ve had enough, I’ve had enough, good God - pluck me” which comes right before the guitar solo but I’ve always wondered what this line really meant.
Way off target here, but I’ve always liked the end of Megadeths Paranoid cover. At the very end of the song all the instruments cease except for the drummer, Nick Menza, who is still wailing away at his kit. The lead singer, Dave Mustaine yells “Nick ! Nick!” to which Nick responds “F*ck me running”. I’m not sure if it was staged or they just left it in for fun.
Beck says, “That was a good drum break” after a short drum solo in “Two Turntables and a Microphone”.
And of course, who can forget Archie Bell and the Drells? After all, they can sing and dance just as good as they walk. And they invented a new dance called “The Tighten Up”, in which Archie calls in each instrument, “Aww yeah… Tighten up on that bass now! Okay, Drummer… Tighten it up!”