So it turns out that that official list of Vita-compatible PSP and Minis titles isn't as definitive as it seems. Despite Sony making it impossible to download unconfirmed games onto your Vita directly from the new handheld's own PlayStation Store, you CAN transfer unlisted titles from the PS3 onto your Vita fairly easily.
Simply download the title onto your PS3 and transfer across like you would to a PSP. If early reports are anything to go by, it seems the majority of unavailable games are already compatible with the Vita. Games like Killzone Liberation, Grand Theft Auto: Liberty City and Vice City stories, Chinatown Wars, Echochrome and Flow are all unavailable from your download list, but run perfectly fine when transferred.
An important note, of course, is that while these games are hugely unlikely to cause any harm to your console, unexpected bugs that damage the Vita and appear in games not on the official list are unlikely to solicit much sympathy from Sony, so use at your own risk.