Exactly! When making any avios booking online for a route via London or Singapore, you are asked if you want a stopover and can very easily select one that is many months long even! Here's an example below, easily replicated.

So if this 72h 'rule' does exist it certainly doesn't apply to online bookings, at least not yet. Of course it's just a shame that anything more complicated than a simple one-way or return has to be done over the phone so you are at the mercy of whatever the agent and/or their computer says on the day.