It isn't spelled out in the rules, but if the object ball and cue ball are frozen, then you may shoot straight through the cue ball. The only place I found this mentioned was in the BCA Rule Book on throwing a ball as a legal shot. This is when the cue ball was frozen to the object ball and you wanted to pocket that object ball. It said nothing about angles or shooting away, but it did show a diagram of it shooting straight through at the object ball with side English.
Here is the only rules I found when searching the BCA and APA rule books that had the word frozen in it, except when describing racking the balls.
BCA General Rules of Pocket Billiards
3.38 OBJECT BALL FROZEN TO CUSHION OR CUE BALL
This rule applies to any shot where the cue ball’s first contact with a ball is with one that is frozen to a cushion or to the cue ball itself. After the cue ball makes contact with the frozen object ball, the shot must result in either:
(a) A ball being pocketed, or;
(b) The cue ball contacting a cushion, or;
(c) The frozen ball being caused to contact a cushion attached to a separate rail, or;
(d) Another object ball being caused to contact a cushion with which it was not already in contact. Failure to satisfy one of those four requirements is a foul. (Note: 14.1 and other games specify additional requirements and applications of this rule; see specific game rules.) A ball which is touching a cushion at the start of a shot and then is forced into a cushion attached to the same rail is not considered to have been driven to that cushion unless it leaves the cushion, contacts another ball, and then contacts the cushion again. An object ball is not considered frozen to a cushion unless it is examined and announced as such by either the referee or one of the players prior to that object ball being involved in a shot.
APA Rule
THESE ARE THE ONLY FOULS RESULTING IN
BALL-IN-HAND:
d. The object ball is frozen to a rail and the player is
contemplating playing a "safety." In order for the
"frozen ball" rule to be in effect, the opponent
must declare the ball frozen and the player should
verify. Once it is agreed the ball is frozen the
player must drive the object ball to another rail (of
course, it could hit another ball, which in turn hits
a rail) or drive the cue ball to a rail after it touches
the object ball. If the latter method of safety is
chosen the player should be sure to obviously
strike the object ball first. If the cue ball strikes
the rail first or appears to hit both the rail and ball
simultaneously, it is a foul unless either the cue
ball or object ball went to some other rail.