I have taught in Japan for 20 years and met only one person that was employed at a company that taught business English. I can't say I didn't meet more that I simply didn't know where they worked, but I don't think many attend the JALT or JACET conferences. To teach business English, you can go one of 3 routes:
1. teach it yourself as you collect clients from whatever contacts you can make
2. get hired by a biz English dispatch company, where their clients come to them or you get farmed out
3. work directly for a company that wants its employees to learn English (whether they have their own internal program for it or not)
Option 2 seems to be as insular as you stated. In a way, it's like the eikaiwa biz, teaching the way they want to the clients they can get. Their methods may not transfer to the next place where you'd work, or they may. There isn't a lot of information out there in published research about how such places operate or what sort of methods they use, or what their clients actually feel they have learned. The one guy I have met no longer works at the place, but he described his experience as follows.
Companies decided to tell certain managers that they were going overseas 1-3 months in advance. (Yes, that short a time.)
They might have paid his company or not to take a 2-week crash course (!!) in English.
He was going to do a follow up on how well those biz students felt they were prepared, but I have not seen anything in the past 5 years on this.
The online TOEIC Newsletters have individual case reports of how companies have developed their internal programs or policies about people earning a TOEIC score, and what is needed to be chosen to go overseas. It's pretty puny, about 610. If a company is huge, it might have its own internal English program, but even that is not the norm, I think. People are trying to save money, and they view a lack of an internal English program as one way to do that. Some will pay employees to study, some pay part of the cost, some pay nothing. Most, it seems, just expect people to prepare on their own and at the last minute, or even after they arrive in their new country. Small and medium sized companies have it worst, because they don't have the deep pockets to help their employees learn English, yet these are the companies in greatest number. Japan itself is still insular in this regard, and it's hurting the economy.