What about the upper? If you destroyed your lower wouldn't the upper be broken too? Your whole rifle would be gone and unusable!!!!
The upper isn't even part of the ATF equation since it isn't registered and you can put any upper, SBR or not, on a registered lower. (edit:Bypass beat me to it)
I know this is all hypothetical but I wouldn't risk breaking the law if it happened to me. I would just cry a little and fill out the forms and send off another check.
If you did complete an 80% lower for an SBR it would require a serial number and be engraved but you would just need to put your trust name and #001 on it, so hypothetically if it was damaged you could build another and have it engraved with the same markings as the first one that you could completely destroy and then you and the engraver would be the only people that would know that you broke the law.