I don't know if there's a strong reason why there isn't something like this, or why our messages clear when we sit, but -there is- a sleep place message. And like the sleep place messages it would clearly state 'Whoever is sitting here, .'
I know that we can @lp, tp this detail, but sometimes I forget that it goes when I sit. Plus we'd still get the relaxation benefits plus whatever other benefits there are from sitting especially in places like the cubes I've seen (HabX in particular) where there isn't a coded bed to sit on. It would be the most temporary of the @lp messages and it could clear every time we stand, or we could keep it to show whenever we sit elsewhere. It's the missing fourth @place message!