Nouveau driver isn't whitelisted in /usr/bin/compiz-manager script so compiz refuses to load in such configuration. Maybe it is some arguments to this, but I use it several days and nouveau works perfect. Anyway it could be more simple way to enable compiz at nouveau taking into account that script isn't a config file and will be overwritten at next update.
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I am not willing to add nouveau to the whitelist till it supports more cards
OK, what about a config option anywhere? How it should be enabled by user, in other words? For now I see just one way — modify compiz-manager script every time after updating corresponding package. It is not very easy and intuitive to find this way.