I am not sure a legal seperation will have an effect on a modification. The issue is if the mortgage has two people on the mortgage both people stay on the mortgage until the loan is refinanced or one of the people dies. A divorce does not change the people that are liable on a mortgage, so a seperation will not change the mortgage liability. If there is a reduction in income, illness or disability a modification can be done but it is unlikely that a seperation will do what you want.
You can try and apply for a modification using a seperation as your hardship, but not sure it will be approved.
You can do a loan mod with out the separation, you just have to talk to your lender. You need to stablish your hardship as income hardship. And be patient on the phone average around 2 to 3 hrs, if someone got it done, for less time, good luck, plus a lot of transfering of calls to other department.