So I’m assuming you are opposed to having your form users create a site membership?
I’m not too sure that’s a barrier. You could always present it by saying “sign-up for a FREE membership…and start submitting”.
Then you can design the sign up process in a way that those creating a membership will self-validate; i.e. the membership will create itself once they submit the registration form - no need to validate e-mail addresses. At that point you can redirect them back to your entry form.
Maybe a 30 sec process?
The advantage then will be that you can associate form entries with your users.
Just a thought.