You're still OK to transfer the lower to the Trust after the Form 1 is approved. I've gone back and forth on this answer, but as long as the Trust owns the lower before it's converted to an SBR, you should be OK. The Form 1 MUST HAVE BEEN SUBMITTED IN THE NAME OF THE TRUST or you'll have to build the SBR personally and then do a Form 4 to transfer the completed SBR to the Trust.
In other words, as long as the lower is JUST a lower, it can be transferred to the Trust using only a background check. Once the lower is assembled as an SBR, then it's an NFA-regulated item and must be transferred via a Form 4 (and a background check under Colorado law). In order to build an SBR, the Trust building the SBR must own the lower to be converted. The Trust must also obtain approval from the ATF via a Form 1. Whether the Trust actually owns the lower BEFORE the Form 1 is approved shouldn't matter.
Clear as mud?
Sorry. I didn't write these laws, I'm just doing my best to unscramble them.
Rod