The NXP SPI->UART chips are fairly inexpensive, a dual UART (SC16IS752) is about $2.50. Even the DIP version of SC26C92, another dual UART, is about $5.00.
USB is probably the way to go though, unless you need low latency. It's certainly going to be easier to get going, but you'll end up with a bit of a mess of dongles and hubs. But without support or documentation at launch for the SPI port it's really the only way that's going to work for now.
I've had this same idea in the past, connecting a bunch of serial ports to an OpenWRT router as a console server. They rarely expose SPI or even enough GPIO though, so never got anywhere with that. I like your idea, but I'd want to see a clean implementation using SPI, so I'll be patient for now.