What would be the process to approve a minimal, standardized flight recorder for commercial balloons? According to this article about the recent balloon event, the NSTB will try to interrogate personal electronic devices for any related data. Since some of these mentioned in the briefing do not contain cameras, they may look for accelerometer, gyro, and altimeter data that may have been captured automatically. Chips for these are commonly found in cell phones, tablets and other personal monitoring devices.

Standardization, performance and reliability are only some of the things that a NTSB-recognized voluntary flight recording device would need. Is there any mechanism or regulatory pathway for a standardized yet voluntary flight recorder for balloons, or would it have to be all-or-nothing - required, regulated, and therefore in the end substantially more expensive?

According to this recent WSJ article:


  More than two years ago, the NTSB urged tighter regulation of balloons carrying passengers. Without such changes, the safety board said in a April 7, 2014, letter, “the potential for a high number of fatalities in a single air tour balloon accident is of particular concern.”


Currently, passenger balloons do not file flight plans (see this question and associated answers), and while transponders for balloons may exist they are not required, nor is any passive reflection enhancement for radar.
As Ron mentioned, if you want to produce a "standardized" voice recorder for balloons you'd be complying with FAA Technical Standard Order (TSO) C123 (currently revision C).

Along the same lines if you wanted to make a "standardized" flight data recorder it would need to comply with TSO C124 (and you would have to figure out what data would be useful to record, and how to get that data to the recorder). 

If we're not sticking with an established actual standard then the next best thing are the personal portable electronic devices used in the flight - particularly if the balloon operators are using something like Foreflight, Garmin Pilot, WingX, etc. on a tablet. The NTSB lab has a good amount of experience in recovering useful data from these devices, and they can be almost as useful as an actual flight data recorder. Combined with an onboard camera (say a GoPro) capturing audio/video of the flight you can make an excellent reconstruction of events (and since many balloon operators offer their customers a flight video there's a good chance the cameras are already installed...)



It's worth noting however that none of these devices improves the safety of a given flight: CVRs, FDRs and even cockpit video are forensic tools used after a crash or incident to help us understand what happened, but their value is after an incident, and thus usually after loss of life.  

The other NTSB recommendations (including carrying transponders) are aimed at actively improving the safety of individual flights, and may prevent incidents.