The plan is to have a custom built Bluetooth device, which is not part of the standard bluetooth profiles (https://support.apple.com/en-us/HT204387), and also we can't go the BLE way (not my decision) and pair it with either an android or an iOS phone. It works OK for android of course, but cannot be connected to from an iOS phone, due to restrictions of Apple.
I understand we should enroll to the MFI program and have a license for our device so an iphone can be paired with it and a connection can be established. My concern is that how can I be sure that it will work as there seems to be no opportunity to try it without enrollment to MFI program.
Any help would be appreciated as I'm quite a rookie in this topic. Thanks a lot.
You must apply for MFi program just to get access to the Bluetooth APIs that you need. There is no other way without bypassing Apple's legal requirements.