Overloaded adjective

I've seen some very informal usage of English. People wrote anything, perhaps ungrammatically, and turn it into an adjective, like...

"a you-must-know-who-he-is person" and "a should-have-been-well-done task".

That might be some sort of informal usage, but is there any rule governing such? I've seen some similar expressions by native "writers" (I mean, on their blogs) who try to utilize the flexibility of English.

Wonderful English. Creativity is all!

I think this is done pretty often in English. For example:

A "hard-to-reach number", a "tough-to-solve problem", etc.

In writing you need the dashes to indicate that the phrase is an adjective, but in speaking, you use a certain hard-to-describe tone of voice to say these non-single-word adjectives.

Nice.
It looks as if you can take any 'that'-clause (number *that is hard to reach*; a person *that you must get to know*), drop the 'that' plus any assoc. 'be' verb, and treat it as if it a regular adjective (cf. a coat that is blue = a blue coat).

It may be used informally, but I hear this a lot in business talk.

***I think that this is the modern way of creating new adjectives in English.***
The previous way was to either borrow a word from another language, or coin one from Latin or Greek elements, but this method kinda shows that that period of activity in the English language is at least ceasing if not already having come to a close.