Oh wow, that's definitely some over achievement bonus points if you want to do something for both. It's currently never come up with an artist wanting to do something for multiple series but I don't see why not. If you create something and share it here, I'll collect it after the 25th and include it at the end of the hub series with links back to your series.
Yep, it's still open to the community to join in if you'd like to participate. The artists that help organize and participate in this year's collab are locked in and set. This is strictly a way for the community to join in at the tail end of the event if you'd like to. It can be just a postcard or a whole episode around your postcard, like the example in the OP. All the details and such are in the OP.