Can't help you re the pony tail, but I have a Boreo (the male version of the Borea) used for cycling and it is comfortable and stable. I won this in a competition and would not have bought it otherwise. My bike lights are on the bike, not my head.
One disadvantage of Boreo/Borea for caving purposes is that it is full of big slats/holes on the sides which need to be blocked up to stop water getting in. Also the outer plastic is flimsy which has been compensated for by a thick expanded polysytrene (?) inner shell. This thick inner foam interferes badly with attaching gear to the front/rear/sides of this helmet like lamps/torches/battery and the outer shell is so flimsy that drilling holes into it will likely lead to cracks developing. The other problem is the curved shape of the outer shell which makes attaching something flat quite a problem. The front of Boreo is at an angle of about 30 degrees off vertical which means a lamp attached here in the familiar way would point upwards - which is not exactly helpful in a cave or anywhere else. The rear of the Boreo is near vertical which would be optimal for a battery pack, but it is still curved.
These issues apply to all current models of Petzl helmet. I have an adapted Petzl Elios for caving which is the wobbles-all-the-time kind. This is because the height of this helmet (when stood on a table) is an inch or so less than the Boreo, so there is simply a lot less material to grip the sides of your head with Elios. Again, it has slats to let the water in. I blocked these up by removing the foam inner shell and cutting pieces of 1mm PVC sheet to size, softening them with a blowlamp to the curved shape of the inside surfce of the helmet, then gluing them in, and finally replacing the foam shell inside the plastic shell.
Nothing surpasses the typical building site and coal mine helmets of the last century in terms of ease of adaptation for caving: adding a caplamp/battery to them, and obtaining stability by attaching a decent webbed adjustable chin strap. These mining ones had a flat vertical face at the front for attaching a caplamp securely and pointed at the correct angle so the beam points slightly down towards the floor.
The disadvantage of the building site helmet is that it doesn't have the thick foam inner shell, so if it's hit by a rock and the rock smashes the outer shell then there is only a layer of fresh air to stop its onward journey into your head, which is the only reason I shelled out (pardon the pun) for the Elios.
The desired features of an ideal generic helmet to adapt for caving use are:
1) outer shell made of robust hard plastic that can't develop stress fractures from adaptation or underground use (i.e. being banged and abraded on the passage roof frequently and having a lamp fitted to it)
2) has an inner shell made of thick foam to decelerate any rock that breaks through the outer shell
3) vertical flat front area for attaching caplamp easily and at the right angle
4) vertical flat rear area for attaching battery pack easily
5) no holes or slats in the outer shell that let water in or reduce the protection from falling rocks
6) inner cradle to fit well to sides of your head so as not to wobble around
Boreo addresses points 2 and 6 above, but I feel it lacks all the other caving-friendly features.