What would he have made of the modern "crotch around the knees" jeans sported nowadays by "yoof", one wonders?

An interesting question. What would Ewan have had to say on the subject of very loose trousers? Well, we know that there were situations where he favoured trousers that could be easily removed, because on the album Songs from Robert Burns' Merry Muses of Caledonia he sings the song, Had I The Wyte She Bade Me which includes the lines:-

She straik't my head, and clapt my cheeks
And lous'd my breeks and bad me.
Could I for shame, could I for shame,
Could I for shame deny her

Now, it is not entirely clear what is going on, but it seems that Ewan is in a situation where he requires a young female to assist him in the removal of his trousers - though exactly why he needs them removed remains something of a mystery. Ever a generous, giving character, Ewan is thankful for this help and tries to find a way of rewarding her for her help for as he goes on to sing:-

She bad me lye beside her:
I pat six inches in her wame,
A quarter wadna fly'd her;
For ay the mair I ca'd it hame,
Her ports they grew the wider.

I hope that you find this answer helpful to your enquiry.