The trapeze mechanism on the F-22 actually refers to the system by which the sidebays open and launch WVR missiles. As for the J-20's weapons bay, if you look at the sliding section, it's actually quite similar to the F-22's lower weapons bay wherein there's two rear slanted sections, probably to reduce RCS and to improve aerodynamics when the bay is open.
Compare to the F-35, where the slanted sections either don't exist or are more a function of the bay than the bay ending.
Compare to the F-35, where the slanted sections either don't exist or are more a function of the bay than the bay ending.