Why would it be anything to with Island Records accounting? You may be forgetting that for about a year in the early 80s Kid Creole and the Coconuts were the coolest act in the music business.

How about Carol Vorderman as a backing singer! On the record Teenage Kicks by Dawn Chorus and the Blue Tits. Would I lie to you?

http://en.wikipedia.org/wiki/Carol_Vorderman

...the most bizarre collaboration I can think of lasted for a whole album; Dutch proggers Focus in cahoots with trouser-splitting warbler PJ Proby! Has anybody heard this one, dubbed 'Focus Con Proby'??

quite often tend to be a result of whoever is using the studio next door at the time?

Hence Simple Minds ended up singing backing vox on Play It Safe by Iggy Pop, and, er... lots of other really brilliant examples I can't think of right now but which are definitely real and not made up or anything.

Frankie Goes To Hollywood on ABC's SOS and Jesus & Mary Chain on Erasure's Drama

Perennials on backing duties for whomsoever on TOTP way back when. Actually pretty darn good, but always a tad incongruous on the eye. Also appeared on stacks of late 60s recordings.
Sue and Sunny also of similar territory, albeit slightly less dated look. And not inconsiderable bosomly embonpointage, I recall.

I used to believe that the Nanette Newman who did backing vocals on The Rolling Stones 'You Can't Always Get What You Want' was the well known celebrity married to Bryan Forbes the film director, but sadly, according to my extensive and thorough research on the subject (Wikipedia), it's not true. It was actually a Natalie Workman, who used the pseudonym because she didn't have a work permit.

Delia Smith did really bake the cake for the cover of Let It Bleed though, thankfully.

..but still surprising after a time not listening:

I was listening to Squeeze the other day: Elvis Costello produced Tempted, but he's all over it vocally like a rash. And then Black Coffee In Bed, with Costello and early 80s blue eyed soulboy Paul Young on backing vocals.

There's a Dusty Springfield track 'What's It Gonna Be' which features the unmistakable sound of Dionne Warwick on backing vocals.

is still able to suprise me when Jagger's voice appears on the chorus.

Nothing particularly unlikely or surreal about it. It was pretty much de rigeur at the time for rock/pop acts to ginger up their sound with a brass section and/or a couple of female backing singers (preferably black). Presumably Afrodisiac (featuring future Soul II Soul singer Caron Wheeler) were busy at the time what with working for The Jam, Madness, Elvis Costello, Dexys etc. so “the 2” had to settle for the Coconuts. As David points out they were a pretty hip group at the time; The Face, then at its peak as the “style bible”, seemed to have them on the cover about every other month.
In the late ’60s/ early ’70s a lot of rock records made in Britain, featured some permutation of Doris Try, Pat Arnold and Katie Kissoon on b/vs. Doris is no longer with us, but Pat and Katie are still going strong and recently backed up Roger Waters on tour. Katie Kissoon has been on various records/tours by the likes of Waters, Van Morrison, Eric Clapton, Elton John, Stevie Wonder, Pete Townsend etc. but is more well known to the public for her pop/soul duo with her brother that had a few sugary hits in the early ’70s. Like this one

As far as the Island connection goes, I imagine they probably didn’t know who to book and asked their record company to suggest someone. And Kid Creole’s girls were, I presume, seasoned professional backing singers. Rather than actual “coconuts“.

What artist recorded a single that features Sandy Denny & Peter Gabriel on backing vocals? Then added Robert Fripp on guitar, Phil Collins on drums, Keith Tippet on piano and Percy Jones on bass for good measure.

Charlie Drake. "You Never Know"

Was Michael Jackson's backing singer on the Bad tour but I'm not sure if she actually appeared on the album at all.

... Jackie Bird, BBC Scotland TV newsreader toured with Weller/The Style Council as a backing vocalist in the 80's...

When you hear them back to back it's fairly obvious, but quite a lot of people never realised that the BVs on both this. . .

and this . . .

were by the two wacky hippy geniuses who gave us this:

(I hope the non-embedded links work OK for the divsharically challenged.)

But I am guessing Mark Kaylan and Howard Volman, Turtles, Zappa and T.Rex. (Hope better luck than in the sprogfest)

Long before they duetted on "Don't Give Up", Kate Bush provided backing vocals on "Games Without Frontiers" and "No Self Control" on Peter Gabriel's 3rd album - the so-called "melt" album.

Being something of a Zappaholic I was surprised to discover only recently that the ladies enjoying the benefits of "Dinah-Moe Humm" on Frank's Overnite Sensation track of that name were none other than Tina Turner and the Ikettes.

I seem to remember when she first emerged she had been a back-up singer for Patrick Hernandez.

That may have been culled from her resume.