Why not? If she has the time available she has the choice between an extra £40 or an extra - nil. If you're a regular you'll know whether you want half or a full hour with the girl.
I wouldn't leave it until the half hour is nearly up to ask about extending though.
...because the WGs have their rates and it is non-negotiable (as it states on AW)
As for the above the WG may view it solely as being cheated
and not an extra
Finally it is possible that a punter may change their mind on the length of time they will stay with a WG, punters who visit a regular are not immune from this. I only made mention of 'regular' because I think one is more likely to get a part-refund with a regular than with a WG one is visiting for the first time and/or hardly at all.