According current technology, Beryllium foil cannot reach nano thickness level. CVD is a very common technology. There are a lot of toys or electronics surface shining, which benefit from CVD coating. Its coating thickness is normally micro level. Hifiman have already use nano meter thickness diaphragm, and we don't have any plan to come back to micro.
Have you sampled diamond drivers from Bowers & Wilkens or SEAs or Accuton and tested to see if the technology can be used in headphones?
For example, these OEM diamond drivers used in loudpseakers:
https://www.madisoundspeakerstore.com/diamond-dome-tweeters/seas-excel-t29d001-diamond-dome-tweeter-matched-pair/
https://www.madisoundspeakerstore.com/hard-dome-tweeter/accuton-d20-6-3/4-diamond-dome-tweeter-89.5db-d20n-6-31/
Obviously these are tweeters, only 20mm, but there are bigger diamond drivers. For example Raidho D-1 and D-2 speakers (a Danish brand) uses 115mm diamond drivers for their woofers. So I know much bigger diamond drivers are available for OEMs, easily big enough for a full headphone driver.
Also my question was really implying whether or not Hifiman has interest in making a high end electrodynamic headphone? Obviously it's cheaper to use whatever material you currently have versus beryllium or diamond for planar based headphone.