I don't know about a "cosmic plane" or "orphic source," Tony,
but your basic point, with exceptions, is well taken. We might add that for over ten years after he came to Hollywood, Welles was desired as a leading actor, even a romantic star, at least a hypnotic villain, in many a Hollywood vehicle. The studios would much rather have had his services as a performer rather than those of the all too rambunctious director or writer of exotic screenplays he wanted to be.
It could be argued that, though he had an excellent eye for talent, the actors he cast in pictures where he did not appear were either not yet stars, character actors well past their prime, or foreigners not yet well-known to American audiences. An Orson Welles' film seemed to require the presence of an Orson Welles.
On the other hand, a fairly long list of more conventional "studio pictures," could be drawn up that might well have had the name "Orson Welles -- Check loan-out possibility" pencilled-in for a part on early screen drafts. For instance, Welles would have been ideal for the part of Dimitrios Makropoulos in A MASK FOR DIMITRIOS (1944), a follow up to Welles' adaptation of Eric Ambler's JOURNEY INTO FEAR (1942), in which he had already taken the part of recurring Ambler character, Colonel Haki. Welles essentially would play the part of a Post-War Dimitrios as Harry Lime in THE THIRD MAN (1948), and as I've speculated before, the plot of A MASK FOR DIMITRIOS is much like an inside-out MR. ARKADIN (1955).
If Warner Brothers had been able to borrow him, if he had been intrigued, if he had felt the need, Welles might have played both Colonel Haki AND Dimitrios! That's what the Studios, in those days, would have considered "getting their money's worth." Without all the messy legalities of dealing with "Genius Director Orson Welles and the Mercury Theater."
For another example, think of the advertising campaign which could have been mounted if Welles, in GILDA (1946), had played George Macready's Nazi rotter -- sword cane and all -- in Rio, opposite Wife Rita Hayworth and Glenn Ford. Welles was already in negotiations with Columbia Studio's Harry Cohn for the picture that would become THE LADY FROM SHANGHAI (which itself may be seen as a negative, dark subsequent re-telling of GILDA, with Welles in the Ford part, Hayworth now a jaded blonde wife, and fascist Everett Sloane as Ballin Mundson, wielding TWO canes, come to America). Once again, we have a tie-in to an earlier Welles project, this time with his aborted Brazillian epic, IT'S ALL TRUE. Perhaps, some of the South American footage lying in RKO's vaults might have been utilized as part of the deal. Rita Hayworth had newly borne Welles' baby daughter, Rebecca, which the fan magazines were full of. Pure gravy for PR Men. And I've read that Charles Lederer, Welles' pal, husband of his first wife, helped Ben Hecht doctor the script for GILDA, and that Welles had input on the finished work.
Enough, then. I invite others to suggest pictures that might have been fashioned with Welles in mind, pictures which physically similar actors played in, for good or ill. Welles was no dunderhead like George Raft, turning down masterpieces, but I'm sure a number of producers said in vain the famous line, " See if you can get me -- " I'll invite wellesnetters to make some dream suggestions for pretty good films Welles might have been sought for.
Welles was in the American public eye throughout the 1940's, and though his own pictures did not necessarily make money (an understatement), he was a draw, "a hot property." He was considered a commercial asset as an actor well into later life, always in some demand, whereas for most of his career, Hollywood directors, even writer-directors, with the possible exception of a beloved Charlie Chaplin or a brilliantly entertaining Preston Sturges, were ignored by the public. And so was Welles, except that his style was seen as increasingly bombastic, on screen and off, which eventually would work against him.