britbusdriver,
I've heard some of my colleagues bandy this about, but if someone falls over and injures themselves or others, you may have to spend some time post a long duty day filling in endless forms. As far as I'm concerned, passengers can be cretinously stupid when it comes to their own safety aboard aeroplanes. However, there is a strong argument that the airline and you are not liable if the seatbelt signs are illuminated, a lawful command of the Pilot in Command.
If it happens, try this instead.
Bring aircraft to a halt on the taxiway. Set parking brake and advise ATC you will be stationary for about 5 minutes. Call in the Purser/No. 1 and advise them that you are about to make a PA and would like them to carry out the standard cabin secure check just like thet do before take-off. Then get on the PA and lecture passengers about safety, possibility of having to bring aircraft to halt suddenly in case of fire, brake fire, you name it. (Sometimes I think you can justify scaring the bejeezus out of the passengers if they are so complacent as to wander about the aircraft whilst taxying.) Add in some preaching about no point in rushing as they will only have to wait at baggage reclaim for their bags.
Finally, advise pax that you will not be moving again until pax are all seated, belted up and secure check satisfactorily complete.
A slight rant maybe, but I hope you find it a useful idea.
Anyway, it worked a treat when I tried it.