Two fields is possible, of course. The update otoh will only happen when you apply the preference, when the dissector gets the chance to process the keyphrase into the key value. So not during typing. Also it may not be clear from the UI that a key phrase overrides a hex key.
This is also possible, the dissector would have to verify during application of the preference if it's valid though.
Since there is no 'combinatory logic' in the preferences user interface your options are somewhat limited.
