795 Characters, 140 words, but only one sentence:
With the heat of the stage lights and the exertion of her dance routines 
making good hydration a necessity, Onpu was no stranger to performing 
with an achingly full bladder, but the purple-haired child idol's luck 
had run out as she stood in a large puddle whose color complemented her 
amethyst locks, and before a live audience a hundred thousand strong 
with millions more watching the live broadcast and just as she sang the 
climax of the last song in her set, but as she used all of her 
discipline as a performer to avoid breaking into tears of humiliation on 
stage, instead of the crowd erupting in jeers, they erupted in cheers, 
and too shocked to care if they didn't notice her accident, didn't care, 
or only thought it made her more attractive, the young performer took a 
bow.

249 Characters, 48 words:
I'm trying to sleep, I need to pee.
Cute as an angel, she sleeps atop me.
heavy on my bladder, warm around my shaft.
Try to rouse her, she does not stir.
Can no longer hold, Vaginal passage turned urinal.
I empty within her, she mumbles feels nice.

279 Characters, 42 words: 
Doremi:
Holding contest for all you can eat Steak? Her mouth waters. 
Bursting bladder barely beats Competition. "Where's the bathroom?" 
victory is bittersweet, must eat on full bladder. 
Halfway through first plate, her bladder waters. 
"I'm the unluckiest bishoujo in the world!"

280 Characters, 51 Words:
Hazuki
A picnic with Masaru-kun, but too much lemonade.
Need to pee, too shy to ask.
He notices my distress, asks if I'm okay.
Blushing confession, Princess carry to porcelain throne.
Rush into stall, no  time to waste. He gets my panties down just in time.
I tinkle in his arms.

280 Characters, 48 Words:
Aiko
Leon wants to see who has the stronger bladder.
Challenge accepted, magic bars us from the toilet.
Basketball, baseball, soccer, kendo, I smote him in all.
Little man wets himself, no time to gloat.
I rush for the bathroom, prize within sight.
Stuck zipper finally bests me.

277 characters, 43 words:
Onpu:
Album signing, long line of fans.
Bladder filling, can't stop to empty.
Another done, more autographs to sign.
Thighs Trembling, must keep hands steady.
Bladder Bursting, can't stop a leak.
Peeing Panties, can't let composure slip.
Soaked Shorts, photos to be published.

An Omorashi Haiku with added alliterative appeal:
Pounding Piddle Pot
Pretty preteen's pee peril
prodigious puddle