You could created your own custom channel, but you cannot create your own custom firmware or channel store. The Roku HW has a secure boot into it's firmware that includes the Roku channel store. This is to protect our content owners streams as well as provide the optimal user experience for our users.
The only "workaround" I could think of is to create your custom channel and use the ECP to make sure it is launched. Anyone wanting more control, should check the BrightSign products (Roku's sister company) which enable you to create entirely custom digital signage.