I have to say that I used to loop the Bucc quite often. You had to be smooth and precise with respect to pitch control but a loop could be completed consistently and predictably. The entry speed was as for a roll-off-the-top, about 550 KIAS, followed by a 4g pull. As speed reduced and ADD increased, you would eventually get to buffet onset which you held until 20 ADD was reached. Maintaining this, as speed reduced the buffet ceased (buffet onset AoA was a function of Reynolds number, essentially true airspeed) and you had to be very smooth in pitch because the tailplane was very powerful and aerodynamic pitch damping was low such that any sharp aft stick input could result in a departure (as it could during any 20 ADD manoeuvres). As the nose hit the horizon you selected idle and maintained 20 ADD to buffet onset and then held the buffet. It was possible to play the last 30 degrees in order to level off at entry height, albeit about 150 - 200 KIAS below entry speed. The first time I tried it I selected 1/2 airbrake at the top in addition to idle and I levelled 2000 ft above entry altitude so I knew that it could be flown safely.
In addition to the low speed problem of maintaining 20 ADD, the major issue was the high acceleration during the second half. This was because even at 20 ADD the induced drag was low so it did not really handle like a swept wing aircraft whereby pulling lots of buffet/AoA bled energy and kept the speed down. In addition, the entry speed was high so on a straight kinetic-potential energy interchange you are going to be pointing vertically down at high speed with a very high rate of descent; a slightly slack pull would result in a monstrous altitude loss. Ironically, you were better off entering at a lower altitude because for the entry IAS the TAS would be lower and thus your vertical down speed woud be lower. Therefore, I normally entered at 5000 ft.
I used to finish off my Bucc display (which did not include a loop!) with a vertical roll to roll-off-the-top. With practise, I was happy to enter at a minimum speed of 530 KIAS. If I did not make this entry speed I would fly a straight roll-off-the-top, and from an incremental reduction in entry speed during practise I was comfortable to enter from a minimum speed of 480 KIAS. Note that this was entered at close to mean sea level with a light fuel load in a clean aircraft.
I did once work up a display in the Bucc that started with a 1/2 horizontal 8, entering at 480 KIAS and topping at around 8000 ft. However, at such a high TAS you only had to be very slightly steeper than 45 deg on the way down and you lost a monstrous amount of altitude. This sequence also featured a gear down barrel roll but that is another story. Sadly, the requirement for that display never materialised.
And then the story of the Tristar plugged into the Bucc tanker - that can wait for another day.