So, I just coded/tested/benchmarked a new IP-to-country plugin for Active Analytics -- it'll be an optional, third-party plugin for Piwik. It can search the database in either a flat file or shared memory. (The difference is in the 10ths of a millisecond per query on my antiquated test box.)
In hindsight, I wasted a lot of time over-3ngin33ring the solution. I thought it would be safer to sign and check the integrity of the shared memory segment for users in a shared web hosting environment. I later realized this was a non-problem because shared web hosting doesn't offer access to shared memory (due to security and resource concerns). This came as a relief because signing and verifying killed the performance benefit of using shared memory.