Standing by with credit card at the ready!
It's in the pipeline. Should appear as a sales option on the Web site site when it's released.
Remember though, it's not as simple as just ordering a sidecar; your head-cap will need to be modified so returning it for a rebuild will be part of the process.
Since the Super Minis were the first GLVs built with the redesigned head-cap (adjustable hex pin) from the first pre-order forward, it's a logical assumption that it would work as-is with a GLV-3 sidecar if the hex pin and orange ball were removed.
I have no inside knowledge, just guessing based on the similarities between the head-cap design on my Super Mini and 5v GLV-3.