Digital extras I found with nowhere else to go.
There were two web widgets (more commonly known as blogparts in Japan) created in relation to this game. The first was a countdown in May 2009, while the second was a minigame-lookin' thing in June 2010.
Prior to PjD's release, SEGA released a web widget that acted as a countdown to the release date. SEGA killed the .js file pictured (likely due to the death of Flash), but it was archived via the Wayback Machine, and they never deleted the corresponding .swf file. Naturally, I have both hosted on my site now: the JavaScript and the SWF. Of course, the image tells you how to use 'em. I haven't really tested it though - the JavaScript file still has a bunch of Wayback Macine artifacts.
Back when Flash was a thing, SEGA released a web widget featuring Miku's sprites from the song/game. Here's a blog post about it. It displayed four animations at random. Here's an archive of the page with the necessary code - unfortunately the widget is broken. It seems only the main swf was archived, while the individual animations (1, 2, 3, and 4) were separate swf files, which were not preserved.
When major browsers ended support for Flash, SEGA removed all trace of the widget. I think it might be possible to recreate it (everything was made with in-game sprites, so...), but I don't have that kind of skill. If it makes things easier, I've hosted the JavaScript and the main SWF.