I know we've had issues with this one in the past, but as FIM/TeamCity are two prominent examples of it seeming possible and we generally only target 2008 R2 + these days we should take another look.
Custom actions may also have been a problem before because of 2008 certification, but 2012 only suggests the following is not allowed:
Nested install custom actions (type 7, 23, and 39)
EDIT: This would be in the form of a checkbox where people can opt out.
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