I have a question about the noun phrase their previously washed clothes (meaning 'the clothes that they had washed before'). I know we can't say this, but is there anyone who can explain to me why we can't? What is the underlying rule, as it were?
There is nothing wrong with your phrase.
their previously washed clothes
their (adjective) previously (adverb) washed (adjective) clothes (noun)
our previously worn socks
the previously read books
the completely cooked pork
Need more context. If there are previously washed clothes and clothes that are not previously washed, then it's correct.
They put their previously washed clothes away in the cupboard. They put their dirty clothes in the hamper.
But you shouldn't try to stitch sentences together by adding description from a second sentence. That gets you bodice ripper type sentences.
As he stood on the heaving deck he grabbed her yielding body with his rippling muscled arms.
Is there another deck he could have been standing on? Does she have a yielding body and another one that is not yielding? Has he got another set of arms that are not rippling or muscled?