I'm not sure I can answer you very well..but I do know a bit about the TBC.
In the days of David Squibb I had several ex-primary pupils in the TBC and they got to sing in the most amazing pieces and in the most amazing places.
All of these were extremely able boys with wonderful voices so they did do all the big stuff (including three of them one year in the Magic Flute at Glyndebourne).
I would imagine that it is the best who are picked to do things like Midsummer Night's Dream etc. - by the very nature of the beast surely it has to be the most able who do this high-profile work - but I don't ever remember only seeing 'a handful' taking part. I do think it's the trebles who always seemed the most 'noticed' - however, the mixed choir and the Trinity Choristers were always tremendous, too.
I'm really not sure about how things work these days - I do have a couple of ex-pupils at Trinity but they're not ones I was close to their parents so I don't know the answer, I'm afraid.
Surely you can ask these questions of David Swinson when JMNW goes for his audition?