REST endpoint would be an ideal solution for this..
In JSP/HTML, when the user clicks on submit button, construct an XML with form field values and assign to a hidden field.
for e.g. txtFormData = "<mydata><username>testuser</username><password>pwd</password</mydata>"
The target URL of your form should be the REST endpoint address of livecycle.
e.g.
http://localhost:8080/rest/async_invoke/Test_Apps/processing_service:1.0
In your process create a variable named "txtFormData" and receive this input content. Later you may deserialize as XML document and merge it with PDF template. Finally assign tasks to users.
Let me know if you stuck up anywhere with this approach.
You may directly post all contents of your JSP form fields to LC; the problem is you will have to create individual variables for all the form fields.
Thanks,
Nith