I'm thinking of modding several different tables to be these stands. However, since my modding experience is pretty much limited to simple editing of a few file types (itun, xml, objk, objd), I'm not really sure what to do. Just changing the script adds the interactions, but sims can't route to the table. Why is that, and could someone tell me what else is needed?

You could try having the tables point to the code used for the original stands. You should check out Cinderelli's Savvier Bakery mod which basically did what you're trying to do I think but with Ani's savvier seller mod. I think Sandy of Around The Sims also created a few tables that work like the produce stand and use that code, so you could probably see how she did it if you haven't yet.

I finally got a desk to work as a produce stand.
In addition to changing the OBJD and OBJK, I had to also change the _RIG and RSLT to the ones of the produce stands.
I'm not sure if I did it entirely right (the desk has one of each, and the stands have two; and I only put in one each), but it seems to work enough.