@Zee-Man @KelleyBee (not sure who else was involved in this conversation initially, but want to update since I have some new info-
I just finished calling the Standlee Forage Company about their hay cubes. I was told (and anyone reading this can confirm for themselves by calling them, if you like) that they weren't sure why their hay cubes are not labeled for rabbits, since there is nothing in them that makes them unsuitable for rabbits (including the bentonite clay). She said I should be good to go with feeding them (I then told her that I had already been feeding them for about a year with good results). So I believe we have a green light on these cubes.
@robeyw I have not had good success feeding the 1/4 inch Standlee Timothy hay pellets to my rabbits, either. They waste a lot, since they take a nibble or two and then drop them, with a lot more waste than dropping one or two feed pellets. I got a few to like them, specifically a couple young bucks that were bored, but I won't be trying that again. Interesting that yours liked a different brand more...