That was already done when Congress passed the Respect for Marriage Act. Should Obergefell be reversed, any couple legally married in the state or country in which it was done will be legally married in their home state, regardless of whether or not that state has such a law on their books.
True. Congress passed the Respect for Marriage Act , which created a federal law which explicitly legalizes same-sex marriage.
So even if the Supreme Court were to reverse the case on same-sex marriage, the fact that you now have a federal law which explicitly allows it fundamentally changes the legal landscape surrounding same-sex marriage.
Reversing the Supreme Court decision would legally say that there is no constitutional right to same-sex marriage, but such a ruling would not over rule the Federal Respect for Marriage Act.