Here are some photos of a Bentley brush I converted.
The holes are 6 mm. This gives a nice tight fit to the pencil jets. Because the brush back is so much thinner than a Vikan, the jets stick out. I never found this to be a problem as they never touched the glass.
You have to remove bristles, of course. I think I probably removed too many. There was no need to remove the strip in the centre. It's only necessary to remove those immediately surrounding the jet.
You can use one of the holes left by the bristles as a pilot hole to drill the 6 mm hole.
Yes, it worked fine. I didn't find any problems with it being flocked.
Many people say that flocked brushes hold dirt whereas monofilament brushes don't. Just because people say it, doesn't mean that it's true. It doesn't mean that it's untrue either! Plenty of people use both flocked and monofilament and get perfect results. I think it comes down to technique and knowing your brush.
The main advantage is, of course, that it's soooooo light compared to the Vikan. It really does make a difference when reaching at an angle to get to awkward windows over conservatories etc.
The disadvantage I found is that it's poor quality compared to the Vikan and doesn't last as long. But at cost of less than a fiver, who cares?