If he wants filets I strongly suggest doing them individually, not roasting them and then slicing. Your original idea strikes me as best- cut, wrap & mark them at your place, then finish them in the oven on-site. I'd simply season them with S&P or Canada's Best/Montreal Seasoning before searing, then brush 'em with butter and pop them in the oven. Brush them again before serving.
What type of port wine sauce does he want? A glace de viande or demi with port wine would probably be nice.
90 shouldn't be too bad in any event. Should be able to herd them thru service fast enough to get the steaks out nicely (provided everyone get's organized- maybe easier said than done!
).