I've had good success doing this, and have had carryon bags far bigger / heavier than the official allowance. It gets a bit tricky if the flight is busy, as the gate agents often walk down through the queue at the gate looking for big / heavy bags to gate check! I've been challenged once or twice, and just responded with the "but the checkin agent said it would be fine" line and they left it at that

The 6kg rule is crazy - loads of carry-on bags themselves weigh this much before you put anything in them!