Although the P-8A is capable of low-altitude operations, it is designed primarily for use at relatively high altitudes to enable the aircraft to keep watch over large ocean areas and fly as fuel-efficiently as possible.
MAD was crucial to the P-3’s traditional low-altitude tactics. Compared to turboprop aircraft, the jetpowered P-8 is more efficient at higher altitudes, where is also has best use of its surveillance equipment. It can also avoid detection by submarines better that way (engine noise from a low level P-3 can be heard/registered below the surface). One advantage of a drone MAD system could be the ability to investigate multiple target more closely simultaneously, while staying at altitude. P-8 probably can get good results anyway from sonobuoys. So I don't think it is a sensor issue per se, but rather a platform/system optimization issue.
On a plane, the MAD sensor is placed at the end of a boom or a towed aerodynamic device to reduce interference from electrical equipment or metal in the fuselage of the aircraft. A MAD drone would be even further away from the aircraft fuselage and - if the drone is made from non-metallic composite materials - could have better relative performance. The size of the submarine and its hull composition determine the detection range. For detection of the change in anomaly, the submarine must be very near the sensor's position (or vice versa) and close to the sea surface. The MAD sensor works best at low altitudes.
The bomb bay of the P-8A does not contain a rotary launcher, AFAIK.
Bouys are ejected vertically under positive pressure through close fitting tubes