A Crown of Stars: In chapter 22 Misato meets Shinji again. Promptly she tackles him and hugs him, so enthusiastically that she nearly suffocates him. Shinji barely notes that he was saved from Marshmallow Hell only because he had grown a bit since the last time that she saw him.
Shinji found himself tackled hard from behind by someone who began trying to squeeze the life out of him. Or possibly suffocate him; he was spun around long enough for the attacker to renew the attack-hug from his front, and he was saved from marshmallow Hell only by the few inches hed grown since hed last been grabbed by someone this endowed.
Shinji And Warhammer 40 K: In chapter 24 Misato hugged Shinji against her chest and nearly asphyxiated him. Shinji had to pull himself loose from it, thinking that "those bosoms were deadly".
Happens accidentally to Spike in the third chapter of Solar Embrace, Spike's face winding up wedged between Celestia's breasts as their flying carriage is tumbling out of control during a thunderstorm.
Barbara does this twice to Laharl in Just Following Orders. First from behind in chapter 2, then on his face in chapter 3.
Child of the Storm has Harry 'suffer' this once or twice from Darcy, who's largely doing it to troll him. This leads to an amusing scene when Harry, who's had a growth spurt, is now too tall for this to happen and is unsure whether he's pleased about this or not. Darcy, of course, picks up on this immediately.
Yeah, from now on you gotta buy me dinner and ask real nice if you wanna get permission to shove your head in my rack again.
This deviantART has Laharl suffer one from a Succubus version of the authors friend.
She also does it twice in You Got HaruhiRolled!: in the first time, the Alien vs. Time Traveler arc, she accidentally does this to Taniguchi when Yuki knocks him into her. The second time is during the anniversary chapter, where she quite deliberately does it to Fujiwara, in an attempt to smother him. Which, if you know anything about Fujiwara and Mikuru being siblings, suddenly crosses into (quite unintentional, as the tenth and eleventh novels had not yet been written) Fridge Horror.
In the Sly Cooper fanfic Lament of Carmelita, the character Karla Chintzy uses this when trying to seduce Sly Cooper, though fails due to Sly only being interested in Carmelita Fox. She tries this move several more times again in the sequel Dawn of Progeny (albeit as an attack instead of a seduction tactic); first against Guru when she attacks the protagonists after being awoken from cryogenic stasis, then in a sparring session with Sly's son Conner Cooper, then again during a fight with a clone of Carmelita. Each attempt is equally unsuccessful, as she invariably ends up being hit by a counterattack before being able to subdue her opponent.
In the Soul Eater fanfic Winter Waltz, Patti does this intentionally as a way to knock poor, unsuspecting guys unconscious. Naturally, Kid is shocked.
Liberally done in Dreams Scented of Coconut and Apple by Lindsay to Cody. It started off unintentional, but became quite deliberate for her to do as much as possible at every chance as she came to have feelings for him, and admitted to him she loved the feeling of putting Cody's face in between her cleavage.
Lindsay again in Total Drama: Cody's Redemption, also with Cody, for the same reason as above. She is in love with him and is hugging him to her chest to try and seduce him to be her boyfriend.
Hilarity ensues in the Discworld when very tall Assassin Miss Alice Band is forced, by circumstances, to offer a dance to the short and portly Thieves' Guild leader Mr Josiah Boggis. The conversation, on the part of Mr Boggis, is somewhat muffled.
An example in Vale's Underground is Played for Drama. As Raven rapes Qrow for being gay in a vain attempt to change his sexuality, she shoves his face in her chest to get him used to the idea of feeling women's breasts. She did this to him when they were younger as well.
In The Subterfuge Harry's cousin Dudley ends up in St. Mungo's after Harry's accidental magic partially transfigures him into a tree and Snape has to use Polyjuice to take Dudley's place so the neighbors don't become suspicious. Aunt Marge shows up on Dudley's birthday and proceeds to smother Snape in her bosom while hugging him.
Izuku in POWER!!: An origin story is a frequent victim of getting his face pressed against his girlfriend Nejire's breasts, but the crowning moment has to be in chapter 26, where he almost suffocates.
Date A Live: Altered Timeline: After Natsumi heals some civilians with Haniel, Miku gives her a tight hug in praise of her skills much to Natsumi's displeasure, to the point that she starts thinking that she will use Haniel to turn Miku into a frog if this ends up becoming a common occurrence.
Happens at the end of the ParodySlash FicVoyager Chicks Behind Forcefields. Seven of Nine clutches B'Elanna Torres to her impressive chest, saying they'll never have to suffer the Girls Behind Bars trope again. B'Elanna's response?
B'ELANNA(head buried in Seven's cleavage): "Mmmm, mmm, mmmmmmmmm!"
Jaune Arc, Lord of Hunger: When Jaune wakes up in the hospital after his fight with the terentatek, he's subjected to this by a nurse. The nurse was monitoring Jaune's vitals and didn't notice that he was awake when she leaned over his bed to adjust his pillow, unknowingly smothering his face with her bosom. Because he still hadn't recovered from his injuries, Jaune nearly blacks out from being unable to breathe.