IMO, it sounds like a bit much. That's 9.5 hours of steady riding every day, with minimal time for breaks. That doesn't leave much time for eating lunch, bathroom, resting. I wouldn't do it just because it's so time-crunchy. No room for error (flats, mechanical issues). And no sag support is kinda scary to me.
Sounds like a great idea for a ride. Perhaps shorter days riding? Just one sag vehicle along the route each day?

And 15 mph avg is a pretty good guess for 139 miles (for me, anyhow). Is this just for the tri club members, or will there be all levels of riders there (which is scarier, because you KNOW some people will avg far less than 15 and be out there much longer- while some will avg more than 15 and have extra time).

How many days of continuous riding are you talking here?