In short, no it is not a 'legal' routing or however you want to put it. Whether someone gets away with it is another matter.

Diamond club rules stipulate that you must fly the most direct route.

The routing can be done in two flights.

Try asking BA Exec Club to route you London-Dubai-Hong Kong for example. You'll find that you'll probably be told to make two redemptions as there are 6+ direct flights available.

Moving off topic, but another thing that grates me, is when people say "but there isn't availability on X route" as a means for excusing themselves from not flying the most direct route. It's tough. You can't use your miles.

As an example, theres no availability on London - Los Angeles. Does that make it OK to fly London - Bangkok - Los Angeles? Hmmmm!

Rant over