Results for associated views you can see at screenshots:
Source code and complete dll you can download here:
Plugin registration:
Assembly:
Execute step registration:
RetrieveMultiple step registration:
Rollup step registration:
Full registration:
To make possible Rollup message handleable with plugins you should execute following query against organization database:
Update SdkMessageFilter
Set IsCustomProcessingStepAllowed = 1
Where SdkMessageId in
(Select SdkMessageId From SdkMessage Where Name = 'Rollup')
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