Coppola already had one directorial credit under his belt at this time,
The Rain People, a low budget movie.
One of the problems here with
Patton is that it was in production at the same time as two other war movies at 20th Century Fox--
Tora Tora Tora! and
MASH. Robert Altman's movie was fairly low budget, which is precisely why that ex-TV director was given such licence to do whatever the f@#k he wanted to with it.
But those other two movies are, cumulatively, potential studio destroyers if they aren't handled well. As it is both films in OTL went on to become hits, but this wasn't a sure thing. Daryl Zanuck Jr isn't going to trust
Patton with such a junior director as FFC was at this time. In fact people were mildly surprised when Coppola was given such a big property as
The Godfather was later that year!
I don't think any freshman directors were ever given major war pictures to direct in this era. The Hollywoood business model wouldn't allow it, not at this time. You have to go forward a couple of years to
The Deerhunter to see that happening.
Does anyone want to say whether or not they think the idea is plausible?
What, you don't want me making a lame joke that ignores the simple facts of your OP, i.e. that the movie script is already entirely a product of our HGIC*, Francis Ford Coppola?
*
Head Goombah In Charge