Implemented this by setting using the CellActivating and CellDeactivating events to control the AllowEdit setting on the field. Not sure if this is the best way to implement it.

private void OnCellActivating(object sender, CellActivatingEventArgs e) {
    if(e.Cell.Record.Tag != null && e.Cell.Record.Tag.ToString().CompareTo("CheckedOut") == 0) {
 e.Cell.Field.Settings.AllowEdit = true;
    }
    else if(e.Cell.Record.IsAddRecord) {
 e.Cell.Field.Settings.AllowEdit = true;
    } else {
 e.Cell.Field.Settings.AllowEdit = false;
    }
}

private void OnCellDeactivating(object sender, CellDeactivatingEventArgs e){
    e.Cell.Field.Settings.AllowEdit = false;
}

The only problem is that when a cell has a drop down in it (XamComboEditor), if it's enabled, and I mouse over other fields in that column, it paints a drop down arrow. If I click on a another cell in that field that's not enabled, and then mouseover that cell, the down arrow is removed.