I can take your already excellent idea one step further. Put a raised platform in the beetle bin, something with a flat or very slightly cupped top, then put your pupa up there on a small amount of bran, the beetles will "hatch" and will wander off the side of the platform into the beetle bin below, no pupa in with larva or beetles.
I may try that! It would eliminate a step in the process. But would the pupae tend to dry out elevated above the substrate?