Ménage à 3: This trope is a running gag in this comic (borderline NSFW links follow):
It's played for laughs, of course, but the events and Zii's reactions could easily be seen as fuel for, if not straightforward instances of, Les Yay. Well, Zii does eventually turn out to be bisexual, so it's not surprising if she doesn't seem to mind.
Later, Zii uses the effect as a deliberate tactic against Gary. (strip #51, September 17, 2008, NSFW.)
For another example, see Sonya on Zii when the former has stirred up some drama from Yuki.
Senna executes a marshmallow attack on Gary here. It's a relatively mild example, but it serves Senna's usual purpose of keeping Gary off balance.
Later, Kitty dreams what are implied to be Past-Life Memories. He is a soldier wounded in a heroic rescue mission, and as he dies, his nurse bemoans that he died so young, he'll never know a woman's body. The next panel shows him tucked snuggly between Jamie's prodigious breasts.
Grrl Power kind of inverts and lampshades the trope here, as Halo rather walks into the problem, and Anvil speaks from experience.
Now you know why I like dating tall guys.
Delilah the Valkyrie does this quite often to her reluctant groom Weasel Boy (and at time to others as well (as Lust Lass found out).
In this strip of the Touhou Projectdoujin4komaLife of Maid, Chibi Yuyuko drinks the milk of a cow that defeated her in a battle of Moe (we wish we were making this up) and promptly turns into her non-chibi form, complete with large breasts, before giving Youmu a great big hug. This turns out to be a dream (a good dirty one, to be precise) that Youmu is having.
ThisEvil, Inc. episode is a special case. There the hell is mainly from being crumpled.
In Blooming Faeries, Princess Heather uses Marshmallow Hell offensively against Albert when she realizes that he can't give her commands when he can't speak.
Sequential Art: In this strip, Leonard winds up getting squished between the wall of a bubble that was protecting the main cast and Kat.
In Tales of the Questor, Kestrel does it to a fellow bar patron who briefly mistakes her for male. Another patron hopes for the same treatment but gets a barstool to the head instead.
For another R.H. Junior example, in Nip and Tuck, Janine and guest star Bandit get pressed together in an absurd accident with a revolving door. When she notices where his muzzle is, she asks how he's enjoying the local sights.
Played for laughs in Shotgun Shuffle when Ellie nearly smothers Quinn with her chest while they're fighting. Quinn has to take a second to gasp for air after she knocks her off.
Peter enjoys this with Sarah in her were-form. She goes from a normal size human with small breasts to a giant 10ft tall werewolf with breasts the size of her head.
Cherry Li deals with a double Marshmallow Hell when she gets in between Sarah and a werefox on page 152.
Baker, from Aminal Control, gets caught in Sarah's breasts and taken for a run.
, who thinks "Hey... I could get used to this..."