In conducting research for my Father Serra's Legacy novels, I've encountered numerous references to a friar's "license", sometime saying the license "expired" on such-and-such date?

What on earth was this license?

All the friars were ordained priests so why would any of them need some kind of license to preach? Are priests ordained for life? And does not the ordination give them the authority to conduct holy rites in accordance with Catholic doctrine?

All priests need faculties to preach, hear confessions, celebrate Mass publicly, witness marriages, etc. It could be referring to that. Ven. Solanus Casey is a good modern example of a priest simplex who was never permitted to preach or hear confessions.

Although simplex priests no longer exist in canon law. (That doesn't affect the OP's question, though. My guess is that a missionary needed some sort of permission to go about his work.)

originally, only bishops could preach. The two major orders of friars (not all of whom are priests, btw ... St. Francis himself was not a priest) had to get permission from the Pope.

ALL friars serving in California were ordained priests.

I am gaining information that indicates it was probably permission granted by the College of San Fernando and the Archbishop of Mexico for the friars to conduct their missionary works in the diocese of California.

Right, but again, originally only bishops could preach. Even a priest needed permission from his bishop. Even now a priest needs faculties to preach, though in current law such faculties are basically automatic unless they are taken away, upon being ordained.

This is why in the old days only bishops and Dominicans would have a processional cross at Mass, it was a symbol of preaching

True, but one could become functionally a simplex priest by having faculties removed, no? Faculties aren't an all-or-nothing thing, are they? Is it possible that there could exist a priest who had a celebret but no faculties to preach, hear confessions, or witness marriages?

I suppose so, but I believe (remember that I am not a canon lawyer) that a bishop would have to show pretty good cause to leave a man in that state for an extended period.