I'm wondering how I can specify the target field for a "set field" or "go to field" script step via calculation rather than a statically-defined field. I'm surprised that isn't an option, as it is for other "go to" script steps. Any tips?

There is no way to dynamically set that, no. If you've got a small range of field to consider, you could use if/else statements to determine whice field to set when the desired condition is met. If it's more than 20 fields, it could get cumbersome to write out and maintain that many statements, but it'll work.

Urgh. I thought that was my only option. There are way too many fields for this to be viable. What I'm trying to do is script field changes. I'm not enabling editing in browse mode. So, I can hold the name of the field to be edited in a global field and capture the replacement data in a global field. I just can't figure out how to get the new data into the regular field (hence wanting my target field to be the one captured in the global field). Any suggestions for doing this?