Multi-column sorting: Lock one into ASC

Right now I am using row grouping. This means when you sort, you see a duplicate of the row grouping header which I find annoying. To fix this, I make each column always sort by the "day" column first which is how they are grouped. This keeps the groups together while sorting, but it also toggles the day column from ASC to DESC or vice versa each time. Is there a way to lock in one column to ASC or DESC? This is an example of what I have:

var day_table = $('#days').DataTable({
    columns:[
        {sTitle:"Day",data:"Day",className:"child",bSortable:false},
        {sTitle:"ID",data:"ID",className:"child",orderData:[0,1]},
        {sTitle:"Username",data:"Username",className:"child",orderData:[0,2]},
        {sTitle:"Favorite Number",data:"Favorite Number",className:"child",orderData:[0,3]}
    ]
});

What I'd basically like to do is pass something like: orderData:[[0,'asc'],[1,'']] meaning column 0 always stays sorted as asc no matter what I do.

I can think of a few "hack" ways of doing it but I was hoping there was a real built in solution.