Well, right after posting this it occurred to me that I can just hack my way around the problem like this:

Serial.print("{UI|SET|TextBox1.Text=");
Serial.print(abs(finalValue_mm * 2 / 100));
Serial.println("}");

Certainly not ideal, but at least I'm moving forward again. Any input would sure ...

OK, this seems like it should be a no-brainer, what am I missing?

Why does this work:
UserControl1.SetText("TextBox1", "foo");

But this fails:
testing = "foo";
UserControl1.SetText("TextBox1", testing);

I've spent hours on these 2 lines of code. My head is about to pop. What am I missing ...