Unfortunately, there's no way to indirectly reference a field with an Export Field Contents step.
Best alternative that I can think of, is to put a series of Export Field Contents steps inside an If, Else IF structure so that the script performs the appropriate export field Contents step based on info passed as your script parameter.
Either that, or store all these files in the same container field in different records of a related table so that you only export from the same field in every case, but now select the appropriate record instead of the field...
would it be possible to set the container fields as buttons so that they are set as the active field when you click on them to run the script.
Or as Phil described have each of the containers in separate records in another table. You could then show all the containers you want in a portal with a button next to each one to open it. That way you only have to define one button and write one script.
A portal is only one of many interface options for using the related table approach...
After a bit of thinking time I came back to this and found a simple solution that didn't involve an overhaul to the db! I did a little test and discovered that an onObjectEnter script does the trick nicely! You can then have the Export Field Contents step export the selected field to the temporary path.