I'm not sure if it's possible. To get around it, I've been placing a duplicate backdrop of an Active Object directly behind it, then gradually tell MMF to reduce the semi transparency of the active through numerous events. This is a fiddly work around. I'm hoping someone can tell me that there is a far easier way?
MMF 1.5 used to have an extension that let you adjust the brightness and such, but the dev that made it didn't port it to MMF 2. Actually there were a ton of interesting extensions for MMF back then that I lost when I upgraded to MMF 2.