We say "cabin crew please be seated for take off" prior to receiving a clearance to either take off or even line up. The reasons being, sometimes its a short taxi, large aircraft with lots of pax, 11 cabin crew, busy airports were you get inserted between two landing aircraft etc.
If we did the PA after we got cleared to line up there would be numerous occasions where we occupy the runway waiting for the cabin ready memo forcing go-arounds etc.
I do not think it is inappropriate to use the word take-off on the PA. It is as likely to be misinterpreted by us both, remember its one of the pilots saying it, as it would be to just line up without a clearance. There are no call signs given in the PA, the other pilot can't hear it. It is totally out of context.