If they're going to mandate locking down, lock down the WiFi radio, as that's the part that uses the radio waves. The WiFi radio can be a "black box" with it own firmware, much like on cellular phones, where the cellular radio is a similar black box.
This keeps the FCC happy, because people won't be able to violate FCC rules, and it keeps users happy because they can keep running custom software. The WiFi firmware isn't typically something you want to mess with anyway.
And that's what the FCC really wants The problem the FCC is seeing right now is the modified firmware allows access to frequencies that aren't allowed to be used for WiFI in the US. This is more than just channels 12 and 13 on 2.4GHz, but also on the complex 5GHz band.
The FCC has many complaints already from airports and other entities whose radar is being interfered with by 5GHz WiFi (the band plan is complex enough that channels are "locked out" because they're used by higher priority services like radar).
And you really can't blame the open firmware guys either - mostly because they don't know any better and they only build one binary that works for all devices worldwide. (the available channels on 5GHz vary per country - depending on the radar in use).
All the FCC really wants (and they've clarified it in the Notice of Proposed Rulemaking) is the steps wifi manufacturers are taking to prevent people from loading on firmware that does not comply with FCC regulations - i.e., allows transmissions on frequencies they are not allowed to transmit on.
It can either take place as hardware (filters blocking out the frequencies), or software that cannot be modified by the open firmware (e.g., firmware on wifi chip reads a EEPROM or something and locks out those frequencies).
The thing it cannot be is rely on "goodwill" or firmware that respects the band plan - i.e., you cannot rely on "blessed" open firmware that only uses the right frequencies (because anyone can modify it to interfere).
The FCC has all the powers to enforce compliance right now - users of open firmware who are caught creating interference with higher priority services can already be fined, equipment seized and all that stuff (and that would not include just the WiFi router - any WiFi device like PCs can be seized if they attach to that network). That's the heavy handed legal approach they have. However, they don't want to do that, because most users probably don't realize the problem, and the FCC really doesn't want to destroy all that stuff. So instead, the FCC is working with manufacturers to fix the issue at the source.
The problem lies in the fact that most manufacturers are cheap and will not spend a penny more, so instead of locking out the radio from interfering, they'll lock out the entire firmware.
The FCC mentions DD-WRT and all that by name because their investigations revealed that when they investigate interference, the offending routers run that firmware (and which doesn't lock out frequencies that they aren't supposed to transmit on).