Wass,
You can do a form 1 for an SBR if it is an AR15 lower. All you have to do is register in Eforms once it is up and running and fill in the information for a form 1. You will be the "maker" of the SBR and that is what allows you to complete the form 1. You will have to have the lower properly engraved. Feel free to give me a call or PM if you have questions. I'm about to do another form 1 to sbr another lower.
Its a shame they didn't mention form 4s in the letter. Hopefully they come back online soon. I have a Saker 7.62 waiting on me.