Could you please explain further your need? I have not fully understood what you are trying to complete.

I see, the SubReport has "updating" and "updated" event which you can register to add custom image. On "updating", you may show loading image and on "updated" event, you can turn off it.

<script type="text/javascript">
    subReport.on("updating",function(){
        console.log("The subreport is preparing to update");
    });
    subReport.on("updated",function(){
        console.log("The subreport is updated");
    });
</script>