Ok, nm I WAS doing it right, but missed a brace in my jQuery selector so the value wasn't going over :)
So for anyone else needing this...(using jQuery)
1) Create a HiddenField outside of the RadGrid
2) In the RowDataBound wire up a click event to the buttons
var commentLink = $(".vbCommentLink", '#' + item.get_id()); //Get the linkbutton ref
commentLink.attr('itemid', dataItem["ID"]); //assign the key to a property on the linkbutton
//Wire up a jQuery click event
commentLink.click(function () {
var hiddenField = $('[id$="clickedItemID"]'); //Get ref to hiddenfield
hiddenField.attr('value', $(this).attr('itemid')); //set the value of it