I'm also trying to connect my Mindwave Mobile 2 to an outside BLE receiver. I just tore down one of my headsets and can confirm that the bluetooth module on the MW2 is a GT9664B1 (http://www.gigafu.com.tw/product_view.php?id=53) which according to their website has a CC2564B bluetooth chip inside of it. Here's a link to TI's page : https://www.ti.com/product/CC2564
BLE devs will typically follow the example documentation from the chip manufacturer. We might be able to reverse engineer what the custom service is doing while waiting on Neurosky to reply.