It is possible to access the row indicator header via template editing of the GridViewHeaderRow. However there currently is a bug which prevents the application of template of the header row. We are working hard on resolving it and hopefully it will make this or next weeks internal build.
As a workaround you can apply a custom theme to your grid (from the Themes folder coming with the controls installation) and change the header row from there.
I hope this will work for you and I would like to apologize for any inconvenience this might cause.
the Telerik team