I believe all of these in-house signings were in conjunction with the creators appearing at Megacon. All of the CGC news postings about these private signings indicate as such in talking about coordinating the signings around the time of Megacon. I copied the following from Clayton Crain's CGC news posting about his private signing:
For this event, ALL CGC members — including free CGC members — can submit collectibles directly to CGC headquarters in Sarasota, Florida. CGC will coordinate with Crain to have the collectibles signed around the time of MEGACON Orlando, which has been rescheduled for June 4-7. Many guests have already confirmed their attendance at the popular event.
If the CGC Private Signing needs to be rescheduled due to show postponements or cancellations, travel restrictions or other reasons, CGC will securely store all submissions in its vault until the event can be rescheduled.