The biggest problem with open back orthos is the sound is always plasticy. It's hard to describe but the sound resembles what it might sound like if made of flimsy plastic. It loses solidity, resonates badly due to no loading and generally sounds wrong.
Only the LCD2 and the original wharfedale don't suffer this, even the HE5LE has it a bit and I added some damping to mine to tighten them up a bit. Most other orthos I've heard are vented/semi-open for good reason, the original open ones like the T50 and T30 have this same 'off' quality. This is also one of the reasons a lot of the older models are so difficult to damp well, it's a fine balancing act. Most of them also like close ear coupling or again, the sound quality plummets. They truly are a royal pain to get right and it's not in the least bit surprising they went into a long hiatus with only Fostex quietly flying a small, almost unnoticed banner, and even then for a small niche market.
Orthos need a good dose of costly R&D to do well and even though Fostex did a great job of the driver, they did a poor job of optimizing the overall package and failed completely to exploit an eager consumer market. I guess a good part is that even though efficiencies have improved, they are still inefficient compared to the general consumer phone and altering that was probably something even they didn't want to take on head first so they stayed in the pro audio market where it's not really a problem, perceived or otherwise.
I would say that attempting to modify an existing driver for true open operation will prove frustrating but that shouldn't stop an ortho head from trying. I did for a long time and probably will again when time allows.