One of those very nice but also very expensive folding bikes. The shopper I envision is cheap but good.
I have though about the security issue of a beam. If you drill through a beam (extruded aluminium box section) and insert a tube that runs right through the structure, how much strength would you gain or loose? I was thinking about narrow tubes for rack mountings and wider ones for cable locks. A bracing strut may form a smaller triangle near the bottom bracket but that is a pretty inconvenient place to thread a lock.
If the hubs were commonplace, a monostay with fixed wheel would be perfect, you could just lock the wheel.