Just hook it to the 0x01 Content Type.
An example of the syntax used in our SharePoint Audit suite is as follows:
<!-- Add a new action to the SharePoint 2010 Ribbon bar for each list to show all audit entries for a list Item -->
<CustomAction
Id="Muhimbi.SharePoint.Audit.Ribbon.ListItem.Share.ViewAuditLog.Action"
Location="CommandUI.Ribbon"
RegistrationType="ContentType"
RegistrationId="0x01"
>
<CommandUIExtension>
<CommandUIDefinitions>
<CommandUIDefinition
Location="Ribbon.Documents.Share.Controls._children">
<Button Id="Muhimbi.SharePoint.Audit.Ribbon.ListItem.Share.ViewAuditLog.Button"
Command="Muhimbi.SharePoint.Audit.Ribbon.ListItem.Share.ViewAuditLog.Button.Command"
Image16by16="/_layouts/images/Muhimbi.SharePointAudit/Report16.gif"
Image32by32="/_layouts/images/Muhimbi.SharePointAudit/Report.gif"
LabelText="$Resources:MuhimbiAuditProvisioningResources,ECB_ViewAuditLog;"
Sequence="11"
TemplateAlias="o1" />
</CommandUIDefinition>
<CommandUIDefinition
Location="Ribbon.ListItem.Share.Controls._children">
<Button Id="Muhimbi.SharePoint.Audit.Ribbon.ListItem.Share.ViewAuditLog.Button"
Command="Muhimbi.SharePoint.Audit.Ribbon.ListItem.Share.ViewAuditLog.Button.Command"
Image16by16="/_layouts/images/Muhimbi.SharePointAudit/Report16.gif"
Image32by32="/_layouts/images/Muhimbi.SharePointAudit/Report.gif"
LabelText="$Resources:MuhimbiAuditProvisioningResources,ECB_ViewAuditLog;"
Sequence="11"
TemplateAlias="o1" />
</CommandUIDefinition>
<CommandUIDefinition
Location="Ribbon.Calendar.Events.Share.Controls._children">
<Button Id="Muhimbi.SharePoint.Audit.Ribbon.ListItem.Share.ViewAuditLog.Button"
Command="Muhimbi.SharePoint.Audit.Ribbon.ListItem.Share.ViewAuditLog.Button.Command"
Image16by16="/_layouts/images/Muhimbi.SharePointAudit/Report16.gif"
Image32by32="/_layouts/images/Muhimbi.SharePointAudit/Report.gif"
LabelText="$Resources:MuhimbiAuditProvisioningResources,ECB_ViewAuditLog;"
Sequence="11"
TemplateAlias="o1" />
</CommandUIDefinition>
</CommandUIDefinitions>
<CommandUIHandlers>
<CommandUIHandler
Command="Muhimbi.SharePoint.Audit.Ribbon.ListItem.Share.ViewAuditLog.Button.Command"
CommandAction="javascript:window.location='{SiteUrl}/_layouts/Muhimbi.SharePointAudit.Site/AuditLogViewer.aspx?RequestFrom=listitem&ItemId=' + SP.ListOperation.Selection.getSelectedItems()[0].id + '&ListId={ListId}&Source=' + escape(window.location)"
EnabledScript="javascript:function singleEnable()
{
var items = SP.ListOperation.Selection.getSelectedItems();
var ci = CountDictionary(items);
return (ci == 1);
}
singleEnable();" />
</CommandUIHandlers>
</CommandUIExtension>
</CustomAction>