I was planning to call them "wiretap meta files" and use the .wtap extension by default. I forgot to mention that...
This is just a feature needed for the handover feature I plan to implement.
The handover between two processes requires the new process to read the packets for which there are open transactions of the prior process. The metafiles would be just the mechanism I'll use to get the new process which frames to read. How to write them is a whole different story, somewhat similar to what's done when saving related frames, that needs a lot more thinking and is not related to the many advantages of metafiles. I mentioned handovers because is what made me think of the metafile feature.