Howard Weiner has posted here that apparently the alto, tenor, and bass trombone parts were often played on Bb trombones of different bores.
That's a far cry from saying that Beethoven was writing his alto parts with a tenor trombone in mind. I'm familiar with Howard's paper. A lot of the "evidence" seems to hinge on one source:
(Vienna, 1827). In his trombone method,
Nemetz wrote, "The illustrated bass, tenor, and alto trombone is pitched in
Bb," allowing that "the mouthpiece must be different for each of the three
types of trombone"" (Figure 15). From the position numbers printed above
and below the notes in Nemetz' scales and etudes, it is obvious that he indeed
considered all three sorts of trombone to be in Bb."
The problems here are, one, that this was written 22 years after Leonore #3. Two, it's contradicted by the Albrechtsberger source, which specifically refers to an alto in Eb.
The rest of the "evidence" is strictly conjecture, based mostly on the pitch range of the music being written. One of Howard's contentions is that since some composers wrote "E" for the alto trombone, that they must have been writing for a Bb instrument, since E was usually sharp on Eb altos. I'm not entirely convinced that composers would have that much knowledge of the idiosyncrasies of alto trombone manufacturing.
Eb alto trombones were indeed being made in Germany in Beethoven's time. To suggest that Beethoven didn't know any alto trombone players, as you did earlier, Bruce, and to therefore conclude that he was writing with tenor trombone in mind, is quite a stretch, in my opinion. It's like saying Jay Friedman never saw a Bach trombone because he lives in Chicago and Bachs are made in Indiana.
Did the alto fall out of favor at some point in the 19th century? Yes, I think it did. Does that prove that Beethoven wasn't writing with alto trombone in mind? No, I don't think it proves that.
It's certainly interesting stuff, but I would need a lot more than "Howard Weiner said it in a thread" before I'm convinced that a Bb tenor trombone is the historically informed choice on which to perform Beethoven alto trombone parts.
Today's alto trombone bears only a passing semblance to the alto trombone of the period where they were actively used. So the choice to use one is not based on historical accuracy.
We're not necessarily trying to recreate a completely historically accurate performance; we're usually just trying to do what makes musical sense, and a small bore alto makes more musical sense for Beethoven than a giant .547 bore tenor.