First thanks to the ever vigilant Kevin at www.laobserved.com for the tip about lavoice.org and David Bullock's photos from a 1906 book on Los Angeles. Kevin in particular mentions the Alexandria Hotel illustration. But when I looked at it last night - the proportions looked all wrong. Then I realized that this architect's drawing showed the building to be far larger than the existing hotel, much less the original building before the two annexes were built.
Now I am still not certain which side of the hotel is being shown; the short side matches the window scheme on the 5th Street side, but the long side shows it going clear to the next street, all along Spring. But even what is shown would not reach that far.
Then I remembered that where the Broadway/Spring Arcade Building (which is now being lofted) now stands, was once Mercantile Place - a real street before it became an covered pedestrian mall. So it is likely that when the Alex was first designed, it was going to extend clear from 5th and Spring to Spring and Mercantile.
The only other option is that the long side shown is supposed to be from 5th and Spring to 5th and Broadway. But I will need to do a little more research before I can confirm which is correct.