The IRS rule does not exist yet, because they haven't released the publication as of this date. The interpretation of the law is based on the house resolution that was passed by congress and signed into law by the President.
Basically, when you get the 1098 - Mortgage Interest Statement for the HELOC, you will report/enter that 1098 in the Rental Expenses section where it specifically ask if you have a 1098 to report. Even if you don't get a 1098, you will still enter it in the same place "as if" you did receive a 1098 for the property.
