let's come up with a name for posts where midgets/shotas are hiding under skin-tight clothes as they cling to and have sex with a tall woman

yeah...

post #9708689, post #9188420, post #9920296...

The pixiv tag is apparently https://dic.pixiv.net/a/お洋服赤ちゃん, which is something like "clothes-baby"... But that doesn't work in english.
There's a few posts under shared_clothes rating:e and under_another's_clothes rating:e but it's really, really hard to find them all.

It's a very specific fetish tag, as you can see by just looking at the posts, so just a generic search doesn't work, but I'm drawing a blank in coming up with the name here, because it's so damn specific.

I'd go with clothing confinement as it captures both that there's clothing and that a shota is confined within the clothes. Plus a childbirth connotation (confined to a hospital) as part of the fetish.

sex_under_clothes is what immediately came to mind, but that doesn't account for the required proportions, and I suspect the average user would mistag it enough to be useless.

short_male_under_clothes

because short male is a tag as is dildo under clothes

For now under @AngryZapdos's suggestion I made oyoufuku akachan as a temptag, taking the romanized form of お洋服赤ちゃん.
I'm doing this like we did with kankaku shadan, so that we can at least have a tag name even if we can't figure out something that works in english.

since meat armor is very similar how about reverse meat armor?

This is surely one of the topic names of all time.