Earlier quoted context omitted.
Don't most receivers use a standardized intermediate frequency, though, resulting in leakage that always leads back to the original frequency?
Yes, most radios use standardized IFs. But some high-end SDRs use ridiculously-high sampling rates (like 200MHz), in which case many applications don't even need to mix a local oscillator with the incoming RF. It's pretty amazing.
I never learned much about RF, so this question may be naive. Is it possible to sample higher frequency signals using a 200MHz sample rate, by using a bandpass filter and then deliberately undersampling without an anti-alias filter? If so, how high could you go, and what other limitations exist?