Could Tchaikovsky have meant it as musical foreshadowing of characters that were to come later?
I've encountered this comment on the musical reference more than once, and no doubt Petipa intended Odile's port de bras to mirror Odette's in the window. But it's worth remembering that Tchaikovsky wrote this adagio for the first act, when neither Odette nor Odile were on the scene yet. I may seem musically obvious to us, but Tchaikovsky didn't conceive it that way.
the poor swan vainly trying to attract his attention in the window--which is why almost concurrently with that image, and musically in the score, Odile imitates Odette's 'swan arms' to capture Siegfried's attention.
Most musical recordings of Swan Lake follow Tchaikovsky's score, which differs considerably from the 1895 production. The first-act pas de trois as we know it comes after the waltz and the scene with the dowager queen, and that's how Tchaikovsky wrote it. There is the intrada, followed by an andante. The latter is not used in Petipa's pd3, but is sometimes reassigned to the Siegfried to give him a sort of melancholy solo, usually at the end of Act 1. The three variations and coda we're familiar with follow.
I didn't know that! I think I knew that Tchaikovsky's original adagio was too 'romantic' and I assumed that he then wrote up something new. Was the current adagio meant for the Act I pd3?
Immediately after the pas de trois, comes a pas de deux, most of which has been recycled as the "Black Swan" pas de deux. There is a waltz, followed by the andante, which makes up the "Black Swan" adagio. It leads directly into an allegro, which we know as Siegfried's third-act solo, though the original has far more repeats. Then comes a brief waltz, which I've never heard used in any SL production, followed by a coda, the one usually used in the "Black Swan."
Tchaikovsky's own music for Odile and Siegfried was used by Balanchine in his Tchaikovsky Pas de Deux. Overall, it's probably superior music, but it's nowhere near as flashy or melodramatic as the music we know as the "Black Swan," so I can certainly understand Petipa's reasoning.
The question for you historians is, who was the first-act pas de deux intended for originally? The notes to one of my Swan Lake CDs state, not especially helpfully, "two courtiers perform a dance (whose music may be more familiar as the pas de deux often nowadays...given to Siegfried and Odile in Act III)." I've often wondered why Tchaikovsky should have assigned such flamboyant music to a pair of courtiers and far more subdued music for Odile's big seduction scene.