How to insulate flat part of attic ceiling?
You can certainly put the R30 batts in between the 2x6 collar ties, but it won't result in an R30 ceiling installation, due the thermal bridging of the collar ties and the resultant gap above them between the insulation batts. 

I think abetter plan might be to stuff the 6" thick insulation in between collar ties as normal, then lay another layer of 6" thick insulation batts at right angles to and on top of the first course, thereby completely insulating the collar ties and eliminating the thermal bridge.

Also, don't forget to install rafter vents on the sloped portion of the wall so that air from the soffit can flow up to your roof vents and prevent moisture damage inside the attic.