Three things.
Firstly, I'm sorry if I'm being a pain, but I'm still not quite clear on this. If WED exports all entity information when saving to .map format, why wouldn't it import entities using the same format it used to export them?

Secondly, I have searched quite a bit for information on developing WED plugins and my impression is that it hasn't yet been added but is in development. It is mentioned in this post, this one, and this one. I see that these aren't recent posts, but they're the most current references that I can find to a WED plugin. If you know of any information on it, it would be very helpful.
Also you said that it's been implimented since ver 6.7 but the latest version of 3DGS is 6.4. I'm guessing this must be a typo.

Finally, How much does the .wmp format change over time? Is it pretty stable at this point or are there a lot of revisions from version to version?