I have a standard PA (in economy) that I make for loads of 70% or less.... I tell people the % full but advise them that for reasons of weight and balance they should remain in their check-in allocated seats for take-off: As soon as the seatbelt signs are switched off, they are free (and welcome) to move anywhere that they want. I also remind them that if they have a special meal booked, an change seat, it is their responsibility to advise the crew of their new location.
In general, this works fairly well. It is our airline policy that check-in assigned seats must be occupied for take-off and landing, but I find that a brief word of explanation helps with compliance.
There is always one lone individual who ignores the PA, and decides that their personal needs are more important than complying with social courtesies. Quite apart from being selfish, these people are of course stupid - they believe that neither we, nor anybody else notices (we have an empty seat print-out). I take particular pleasure in turfing them out and back to their original seat, at high volume, thereby causing maximum embarrassment.