Hi Andrew, I think you could do this a couple ways. I know you said you wanted to do this without buying the property initially but maybe you could get your mom to agree to seller financing the deal. That way she can earn interest and get paid continuously without worrying about tenants, vacancy, repairs, etc.
Another way is to get the HELOC on the house. Unfortunately I'm almost positive she'll have to be the one liable for the loan. However if she's willing, she can loan you the money for the rehab and you could put in a lot of sweat equity as well as managing any sub-contractors. After it's complete, you could refinance and pay off the line of credit as well as the balance of any current mortgage on it but it's hard to say without any numbers.
If you do this method, your mom would basically be giving you half the equity currently in the house. You could either agree that if the property is ever sold, she could recoup the current equity in the house and you would split anything after that set amount. Or if the cash out refi is large enough, she could take out her current equity that you guys agree on first and anything after would be split.
Hopefully that makes sense and gives you a couple options of how to structure it.