My girlfriend did just that the other day. ^ I was content to sit in the (very legroom-restricted) seat directly behind and witness her giddiness. The young pilot (about our age) pointed out landmarks to her as she took photos and certainly didn't mind the company.

9K accrual on UA applies only on SJU and GUM routes. Otherwise, booking via UA on the same ticket would be helpful if you want to interline checked bags. If connecting via STL, keep in mind that you can stay airside and utilize 9K's tarmac shuttle between C and any other concourse.