It's funny..
I think ' to be in the pocket' has connotations of bribery in English especially where concerning games or bets that are fixed?
Also I was thinking of something along this line:
To have someone in your pocket =
To have power over someone in a position of authority.
In Holland we also know 'Iemand in je zak hebben'
= Llevar alguien en su bolsillo (= igual al inglés)
Which means you know someone very well and can
manipulate him/her very well (or even blackmail them). Which (i.e. the manipulation) is the meaning suggested by Tomisimo in fact as well with 'wrap around your (little) finger, to be devious.
So it seems quite a lot of expressions similar to Spanish with meanings that all indicate
having influence over someone (in authority) yet with different means (by charm, by bribery, by manipulation or by blackmail).