an old expression "enough blue to make a sailor's pair of trousers." seems equivalent to the Dutchman's pants.

Yes, Rod, my Mum used the same expression, but she also used to say " .... a Dutchman a pair of britches," as well, almost as often. So, to me, the expressions are definitely interchangeable.
If those of you who've been following thew "keeping the Dutch from the English" thread think that us Brits are a bit hooked on the Dutch, you should bear in mind that we only fought them for about twenty years or so - most of the time we have co-operated with them and traded with them. The drainage of the flat country of Eastern England was mostly done by Dutch experts, for instance. And, of course, they provided us with a King when we were in need of a new one (William III, married to Mary Stuart after her brother James was deposed - 1687-88.)