Allow custom events to be populated for sub-relations

[#1]

As a media site one of our key metrics is Page Velocity (the number of pages seen after setting the value, titled page consumption in the image below). Apparently participation metrics cannot be populated by channel because it is basically a sub-relation and participation metrics are not available in sub-relations. In the Marketing Channel reports, for example, I can't see which traffic sources is best driving this this most important metric.

m5213_8-9-2011 1-58-50 PM.png ‏43 KB