One thing to learn is that everything is negotiable! You can ask for anything at all while you are negotiating. Some of the things you ask for might be easy to obtain, others might be extremely difficult.
Be aware though, that it is often difficult for a hiring manager to go outside the hiring guidelines that are normal in that company. Sometimes, the hiring manager would need to get approval from several levels of management - and that can be difficult to obtain.
Since you have already interned at this company, do you happen to know if the 30% tuition reimbursement is the company standard? I know in most of the companies where I have worked, it was far easier for me to offer a bit more in salary than to offer something outside the standard benefits.
Do you think the hiring company would agree that obtaining a Masters degree would actually make you a far more valuable employee? One way to determine an answer would be to look at their job postings and see what emphasis they place on advanced degrees. Knowing the answer to this question gives you a sense of the kind of leverage you would have in negotiations.
Be prepared for the hiring manager to say "no". Don't take it personally, that's just part of negotiating. And decide beforehand what your response will be either way. Perhaps this is an extremely important factor, and perhaps you know that other companies will give you salary or tuition reimbursement to cover your tuition, or perhaps not.
Good luck in your negotiations, and in your continued education.