I came across a situation where we need to find the Custom Workflow activity across Processes, to achieve this we can add the Plugin assembly and look for dependencies that's not at all possible in all the cases. 

To overcome this, I had written below FetchXML and executed this in XRMToolBox-->FetchXML Tester

<fetch mapping="logical" count="5000" version="1.0" >
    <entity name="workflow" >
        <attribute name="name" />
        <attribute name="category" />
        <link-entity name="dependency" from="dependentcomponentobjectid" to="workflowid" alias="dep" >
            <link-entity name="plugintype" from="plugintypeid" to="requiredcomponentobjectid" alias="pt" >
                <attribute name="assemblyname" />
                <attribute name="description" />
                <attribute name="friendlyname" />
                <attribute name="name" />
                <attribute name="typename" />
                <attribute name="workflowactivitygroupname" />
                <attribute name="isworkflowactivity" />
            </link-entity>
        </link-entity>
    </entity>
</fetch>