hi Jeremy.
I don't know of any morning mass in SJPP,maybe others will know,-there is one in Le Puy which some consider the start of the
Camino Frances.
its not so much when you start but when you finish-if you are going to Roncesvalles-they don't open till 4pm in a converted barn after you have "checked in" at the monastery across the road.it takes about 6 hours to walk there on a good day-there is a pilgrim mass at 8pm-most people eat beforehand,if you want a pilgrim meal a ticket has to be purchased first at the cafe of your choice.
if walking to Orrison you have lots of time -last year I set off from sjpp at 8am and had to sit around for ages before I could book in.
have a great Camino
Ian