I agree with 'TROLLE' in his previous posting. There is a distinction between a steady state sideslip (as in a cross wind) and a forward slip, which is the technique used on light aircraft to lose excess height during approach.
I believe that the original posting on this thread was assess the relative safety of getting a swept wing transport into a forward slip in order to get height off.
The B777 flying manual specifically recommends setting up a steady state side slip in crosswind conditions and a max bank angle of 5 degrees is quoted. In limiting conditions ( 30-35kts crosswind) around 10 degrees of drift needs to be ruddered off and a control wheel deflection of 30-40 degrees results in order to keep the bank angle to approx 5 degrees. Therefore a sideslip manoeuvre IS permitted on swept wing aircraft, under specific conditions.
However,as in any sideslip, there is increased drag from greater control surface deflection and spoiler deployment on the raised wing, plus a loss of vertical lift component due to the bank angle. All of this results in an increased rate of descent on the B777 (or any jet) which has to be counteracted by a small increase in pitch attitude. If not, the aircraft will go low on the approach path. Within these parameters a small degree of sideslip MIGHT be used in non-crosswind conditions to lose height at constant speed.
If a similar technique were to be used in a forward slip in order to deliberately lose a substantial amount of height, it would be very easy to reach a point where where the rudder input would require full (limited) control wheel deflection in roll, to oppose the rudder . I would guess that this would occur at an offset heading of around 15-20 degrees, which is not a lot, visually. Would you really want to be at a control limit position at a high rate of descent close to the ground? Given the powerful roll/yaw couple of a jet transport, your sudden relaxation of the roll input when it is realised that roll control is being limited or lost could rapidly lead to to an uncontrollable roll-off at low altitude.