Would an EAV model work? I'm thinking something like <form,field,value>. For example: formA,ckbox1,true formA,input1,initial value formB,textarea1,enter your long comment here
You could easily extend this to <user,form,field,value>, or normalize it as you see fit.
This would work great too. If we go this route, we'll keep it simple and use the <user_id, report_id, field, value>, as you suggested, where user_id and report_id are foreign key references.