Druid was actually built to answer arbitrary questions that could not be answered with pre-aggregated data (because it can become computationally intractable), but still needed to be answered fast enough to power interactive dashboards. At Metamarkets, almost all queries are user driven through our interface. Currently Druid is indeed tailored for time-based event data, but it can shard on more than just time. Disclaimer: I am a Druid comitter and work at Metamarkets.

If you're looking for Lightroom integration, mosaic (http://mosaicarchive.com) provides an almost identical service and already has a plugin.

But was mosaic made with love in Nashville? ;-)