@Ars Technica, They report that Beta Testers will get to keep the DLC. But you will have to sign up quickly if you want in.
More information.In a first for a major Bethesda game, Fallout 4 will also allow its PC players to access its DLC content early and for free, provided they become approved beta testers. Bethesda doesn't mean a limited-time free access, where you get into a beta and then lose the content and your progress in it; the developer says that approved DLC beta testers will get to keep all content for free and retain both progress and achievements as the DLC content is patched and approved ahead of retail launch. You'll want to sign up quickly at bethesda.net.