I'm a little surprised that a SP guarantees she'll keep half hour ring fenced for you when you're (a) you're a new client and (b) someone else, e.g. a reg, might subsequent want that time. I'm not sure any given SP would act any differently if you only booked half hour and then extended. Still, as you say, there's no harm in asking. 
Like I say, depends on how busy they are.
I've done this with a couple of trannies, they tend to take on-the-day bookings anyway because they often get let down by 'nervous punters' (
) .
So if I were to call a girl (any girl really) about an hour before and they were having a slow day, they might well go for it.
Of course, they could even say when you arrived 'I'm sorry, you can only have the half an hour because I just picked up another booking after you', I wouldn't consider it written in stone.
It can work, but depends on the girl and busyness. I might also add, I have a rather winning and charming telephone manner
.
