So here is a first-hand experience:
It is possible to access the Suite Lounge (Non-Schengen, Terminal B) with a boarding pass for Terminal A.
And it is possible to access the Loft Lounge (Schengen, Terminal A) with a boarding pass for Terminal B.

If you departing from Bruxelles, you need to pass the newly-opened central security area to get airside.
Two boarding pass checks happen in the central security area (the first one before entering the area, and a second one before putting your staff onto the belt). Because both A and B-departing pax use the same central security area, you are fine with either an A or B boarding pass.
After you have passed security, you may turn left and get to the Terminal A - with no barriers (except the shopping street :-). No one will stop you accessing Terminal A from that point with a Terminal B boarding pass.

If you turn right after security (direction Terminal B), you get to the Schengen exit immigration checkpoint. With an electronic European passport, you can use the automated gates. No questions asked there. Moreover, there is no boarding pass check. So no one will stop you accessing Terminal B from that point with a Terminal A boarding pass.

Hint: If you are already in Terminal B (airside or arrving) and want to access the Loft Lounge (Schengen, Terminal A), I would recommend to exit landside (going through Schengen entry immigration, baggage claim, customs) and enter Terminal A via the central security area. As explained above you can do that with a Terminal B boarding pass.
Moreover, you can exit Terminal B airside departure area easily, in case you want to do lounge hopping or "misuse" the lounge as arrival lounge. This is not Heathrow or Hong Kong!!!

There is seperate channel to guide you from Terminal B Arrivals to Terminal A airside. But, it doesn't save you much time, because you have to pass security and Schengen entry immigration either way. Moreover, the security staff may want to see you Terminal A boarding pass.