The 'bus is on the ground for around 25-30 minutes at BRU... so its all down to how fast you can navigate BRU airport.
It took me 5 minutes to reach passport control, 3 minutes to find my way out. If there's a transit area - I'm not sure to be honest.
Then... you've gotta go upstairs (if your BP is printed at LHR, this may help), through the B control Passport are control again, then through their security, and then race to the gate. (You do like running don't you?)
Very tight to say the least... and this assumes the plane is ontime. If it isn't.. definatly a no-go. Otherwise.. are you feelin lucky punk?