Sorry, I don't know if I understood you correctly.
You have a form with some input (text) fields?
And the default Values are set through your PHP script, the assigned with smarty?
Why validate them with JS, you can validate them with PHP.
Or do you mean you want to validate the users input with JS?
Then were's your Problem? Write a JS which will do this and save it in your tempalte.
The Literal function will help you with getting no errors through the JS {} Tags.
Or did you mean you want to write this JS function with Smarty in your HTML output?
I don't know if there is any problem with the {} tags, I don't think so, but if you could also save your JS functions in an external file.
I hope I could help you...
Thorsten