I believe that pdftk can fill forms (command line app), and I believe evince /or modern poppler may have some support for forms but have never used it. If you've got a suitable sample form to test with, I'll see what I can come up with later on my laptop.
While I use nether application, I can tell you that both PDFedit and Scribus are still using Qt3. The Scribus website is offline right now but Google shows a few articles on their wiki that might help:
Your first PDF form with Scribus (cache) and
How to enhance your PDF forms with JavaScript (cache). So at least Scribus can use JavaScript with PDF forms, in some form. I don't know if it's enough to support what you need to do though.
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Thou shalt check the array bounds of all strings (indeed, all arrays), for surely where thou typest ``foo'' someone someday shall type ``supercalifragilisticexpialidocious''.