A bit late, had the same problem to solve today. Here is how I solved it: Use jqGrid's events and jQuery
Add or alter the following jqGrid Options when creating the grid:
gridview: false, // If set to true, the event below doesn't fire!
afterInsertRow: function(rowId, rowData, rowElem) {
// rowElem[nnn] column number to compare with (remember Javascript starts indexes at 0, so rowElem[2] selects the 3rd column)
// Adapt the if clause to other comparisions if needed
