It appears as though you cannot pass a Universal IO - String Calculation tag configured as a string to a Script tag.

Changing the Universal tag to a Numeric Calculation, but keeping the value as a string, works just fine, the Script is able to use the parameter's string value. Downside of this is that the value of the tag does not show in the tag browser.

As well, using the legacy Calculation tag, a string can be held and passed to a script.

You can, just not immediately.
Script tags predate String and I/O tags by a long ways. When selecting a tag to be the input to your script tag, the filter is hard-wired to take only numerics. That mostly made sense back when the script tag was introduced. But that rule applies only to the selection filter. The script tag itself doesn't care what you give it.

So the work-around is to:

1. Set the mode of your I/O tag to numeric calculation.
2. Create the expression and save the I/O tag.
3. Link it to your script tag, along with your module.
4. Change the mode back to string calculation.

Note that an I/O configured for Numeric Calculation will force the value to numeric. This doesn't work out very well when your expression returns text.

The work around does work, thanks.

It's just sort of odd, since it also works just fine in the Numerical Calculation mode with a string value.

It's much more useful to pass in and read back string values than Numerics IMO, you can delimit any number of values to read back without having to use source debugger.