Star Soldier Vs. DoDonPachi: Dai-Ou-Jou - Caravan '06 was a mobile-based collaborative effort released in 2006. Developed by Hudson and Cave,
this was made available for FOMA 900i phones. I believe this was the last of the old-style releases before moving on to iPhone/iPad development.
As mobile hardware has changed drastically and both developers' original websites are no longer active, I haven't much information here. From what I gather though, this was a subscription-based
game of some kind. I also believe it was a crossover of sorts, where you could fly as Caesar (the SS hero) through two DDP stages or a Type-A ship through a SS stage. As a Caravan title, it was likely a timed/high-score competition of sorts.
However, long ago I pulled some screenshots from Game Watch Impress. Sadly, beyond those and my speculation (based on an ever-fading knowledge of
Japanese), I've got little for you.