Is it against any rule or custom to wear a suit's jacket with odd trousers, rather than as the suit? Or to wear a suit's trousers with some other sport coat or blazer?

No and no.

Obviously, it’s easier to wear the trousers separately when breaking apart a suit—they look just like any other dress trousers.

Jackets are trickier. Ideally, you don’t want the suit jacket to look like it’s orphaned. In terms of the suit, it helps if the fabric has some texture and isn’t pinstriped. Minimal or no shoulder padding also lends to the less formal look.