How would I software-decode this 260KHz serial stream?
You say that the overhead of interrupt handling would be too high, but I don't think that's necessarily true. A cheap microcontroller may not be able to do much useful processing of the data stream, but it should be powerful enough to serve as an ad-hoc converter.

Let's take an AVR chip as an example: the ATmega328P that serves as the core of the Arduino dev board. At the maximum clock rate of 20MHz, you get 77 clock cycles per bit. According to the datasheet (page 15) it takes about 7 clock cycles to enter an ISR and 4 cycles to exit. 

Assuming you spend half of each data bit in a spin-loop waiting for the right time to sample the input, you're left with about 27 cycles per bit outside of the interrupt handler. Most AVR instructions are single-cycle, so that should be plenty of time to frame the bits into bytes and shove them onto an SPI link to your host processor.

(I've glossed over the problem of detecting start bits. One option is to reset one of the built-in timers at the beginning of every bit, and use the output compare to determine whether the interval between two rising edges exceeds a threshold.)