I don't think I've seen this particular type of crossover before - a character from a TV show not yet on the air makes an under the radar guest appearance on another show.
I'm not talking about back door pilots, or characters created to run for awhile before being spun off. In this case, Will Freeman of 'About A Boy' was just one of the poker players in the 'Parenthood' episode "Jump Ball". It wasn't integral to the plot, and any one-shot TV character could have been that seat-warmer.
Jesse Katims is the executive producer for both shows, and he said:
“When Dax comes on to 'About a Boy' it will be very different. It’s going to be a cameo. It won’t be what the episode is going to be about. We’re doing it in ways that feel real and organic to the shows.”