Currently you can copy a mono plugin to a stereo track, but you can't copy a stereo plugin to a mono track. I understand the reason why, but frankly it doesn't make much sense. If the setting are the same between the channels, just apply those settings to the new mono plugin, it's not that hard. If the channels are different....then a popup window appears asking which channel you want to take settings from.
Example of usage.
I use McDSP Analog Channel and Slate Virtual Tape Machine on each channel. Once I dial in my setting, I want to leave it for each subsequent channel that a copy the plug to when I hold down OPTION and drag it. But if I copy that from a stereo track....I have to go through the whole process of selecting the plugin, copying settings, then pasting settings.
This is a no brainer feature and shouldn't even be an issue. Most other DAWs treat mono and stereo tracks the same.