Wow! I had no idea that my innocent little question would generate so much energy. I thought that the answer would probably be either:
More likely: No rule against it, but not done due to lack of interesting story potential
Less likely: There was a general agreement long ago to not do those here.
My read of this thread is that the first is basically the answer. I think the ideas and concerns expressed here are valid and reasonable. I would interpret them as follows:
1. If you are going to do any crossover, it has to be fundamentally a Lois and Clark story, not an excuse to write for the other universe. (Smallville in this case.)
2. It needs to be interesting and entertaining to the “local” audience and can not depend on a high degree of familiarity with the other universe.
And for something like Smallville, which might have a high potential for triggering an automatic negative reaction, I would add:
3. For certain crossovers, don’t even go there unless you're sure you can bring your A-game.
So, I’m excited by the possibility. For my part, I know I’m not ready to try to tackle anything like this yet. But I still think there is a potential for something in this idea. I really can’t honestly say that I have anything worth developing at this time, but now I know the rules and they seem quite reasonable.
Bob