Hi Darren - it is possible to output all of the attribute data for a table in JSON format using {{feature.featureAttributes}}. The template below shows this and also a few other useful attributes like the clicked XY.
<div class="customCalloutData">
<div> FTR-ID = {{feature.id}}</div>
<div> TABLE-REF = {{feature.tableRef}}</div>
<div> X-CLICK = {{coord.x}}</div>
<div> Y-CLICK = {{coord.y}}</div>
<div> MAP-SRS = {{$root.map.projection}}</div>
<div> ATTRIBUTES = {{feature.featureAttributes}}</div>
</div>
In the case of MRR it will show JSON like this {"Grid Cell Value":58}
It was not possible to output just the value as the "Grid Cell Value" is not working as a column name.
You can add this to a hidden input and read it in JavaScript if needed.
Let me know if this helps.