Can one always drive oscillator inputs with a single-pin oscillator output and bypass the integrated feedback circuitry?
Typically, when external clock source is accepted in place of OSC it is also so stated in the datasheet, and even then there could be caveats. For example, TI TUSBxxx full-speed USB hub controllers would accept 12 MHz quartz or 48 MHz clock.

You can test it with not much difficulty yourself. Route a board for the large crystal, like HC49 can, then just leave cap footprints empty and mount an oscillator dead bug connecting its output to OSC1. 

There is another thing to try. I'm not sure about this part but Microchip USB to Serial converter is actually one of their micros ( PIC18 variety if I'm not mistaken) with some of the peripherals non-functional. After erasing MC code from the flash the part can be programmed/debugged as usual. "MCP2211" part can very well be the same micro; in this case to switch to external clock source you'd have to access conf.bits somehow which is not trivial if code protection is turned on inside the chip.