The basic narrative that Brian was cajoled into returning to a modular composing format, and collaborated in many different details with Andy Paley, acceding to many of his ideas and creations, does suggest that the reinforcing presence of a creative partner in the studio (in this case, two of them) is one of the reasons why "Rio Grande" made it onto BW '88 while "Can't Wait Too Long" is still...waiting.
I wouldn't say Can't Wait Too Long is comparable to Rio Grande. That was a simple, commercial song that Brian continually restructured as he worked on it (just presented very confusingly on official releases), while Rio Grande is a conscious attempt to create a suite of unrelated miniature song fragments, something Brian had never really done before. It's more akin to the montage nature of the way 1970 Cool Cool Water was put together, which was Waronker's main point of reference.
And many thanks for the tremendous breakdown of the consituent parts in "Rio Grande," Will. Do we have a session chronology for BW '88--which songs were done when and where?
The broad strokes, few specifics. The AFM contracts only cover select sessions (see AGD's site) and the rest of the current timeline is pulled from rough mixes or anecdotes.
From the spring to early June '87, Brian and Andy worked on There's So Many, Doin' Time on Planet Earth, Night Time, Love and Mercy, another version of The Spirit of Rock and Roll, Saturday Morning in the City, Saturday Evening in the City, I Feel This Love, and early versions of Baby Let Your Hair Grow Long and Let's Do It Again. That was nearing a complete album - and at that point, at Landy's insistence, there were female backing vocalists on every song. It was... odd.
Russ Titelman entered the project when the sessions moved to New York in June, which is also when the Stack-O-Brian approach to backing vocals started... weirdly late into all of his solo career launch attempts. Those sessions introduced One for the Boys, Terri She Needs Me, Walkin' the Line, Little Children, new versions of Let's Do It Again and Baby Let Your Hair Grow Long, and an early version of Melt Away.
Post-New York is when Goodnight Irene, Rio Grande, Heavenly Lover, Carl and Gina, final Melt Away, and Let It Shine were done. The final song selection was whittled down from there and Hotter was a late addition in '88, followed by Meet Me in My Dreams Tonight, which usurped Let's Do It Again on the album assembly at the last minute.
Somewhere in all of that there were versions of Water Builds Up, Christmas Time, a Shortenin' Bread track, apparently Must Be a Miracle, Christine/Living Doll, and of course He Couldn't Get His Poor Old Body to Move. Probably more! A lot of music was amassed over the course of that project.