Hi Darius,
It is my understanding that you want to create mask for a subsystem in Simulink online and also want to edit the mask. In Simulink online, 'Mask parameters' option is present but 'Edit mask' option is absent.
The reason is that right now Simulink online doesn't support 'Mask editor' feature. Refer to Specification and Limitation of MATLAB online that list down the features not supported by MATLAB and Simulink Online.