I've always politely declined requests from anyone to put up the armrest, POS or not. I'd personally feel too awkward to talk to the FAs about it so I've just dealt with it in the past when this has happened, however you should be able to talk to the FA in private to communicate the concerns. An amusing aside, but the day before my birthday this year, I spent the day leaning into the aisle of an E170 from DFW to EWR, getting battered by the service cart due to the POS in the window seat. Arrived home and had several glasses of Longmorn after that one.