When I've done this in the past I kind of do it in such a rudimentary and ridiculous way that it might be laughable.
Want I do is I'll drag in some drum loops or tap out some drum loops on my padKontrol that fit some of the rudimentary riffs I want to use in the song. I try to make sure that those drum parts aren't too hyper specific so they'll fit over anything I might add on (and if I go too far astray I'll find more appropriate loops or create more of my own). Those all get turned into short Groove Clips and I drag them way down the timeline so they are out of my working area (so I may start off thinking the song will be 3-4 minutes and then drag them to the 5-6 minute mark of the MIDI drum track).
I may also do that with some bass stuff but I usually do the guitar first and really adding a third track just makes things more complicated. However a bassline/riff clip can be linked to specific guit clips for easy editing/structuring.
So as I write the riffs I simply copy/drag the appropriate loop to the appropriate spot. Because they are "Grooves" I can drag them out/in/edit them as far as the riff need.
In the guit track I'll record little riff chunks into the lanes then trim them to beat/measure markers.
Now a I can just drag the riffs and beats around until I get a structure going and insert new/distinct riffs or tags or whatever as needed. All my drum parts are off screen and if I've got a riff that I kind of want to use but it ain't fitting in properly I can drag that off screen. Actually I could copy/drag all the riff clips off screen underneath the drum clips they work with to do easy Ctrl + Select + Drag of sections but I haven't really needed to do that yet.
So really the arse end of my project ends up a clip/beat/riff depository and I write into and build up the song on the timeline before that. If the song starts getting too long I can just Lasso all the source clips and drag them further down the timeline to get them out of the way.
Obviously the more tracks/instruments invloved this process gets more complex but I have always preferred writing with just the guitar and drums first to get the structure and then wrapping/writing everything around that.
We really should have that "Block" style editing I keep hearing so much about in other DAWs though the seems to essentially allow you to segment a project vertically into columns that you can select and move/copy/etc and hopefully deselect elements from. That to me would be an excellent songwriting workflow.
Cheers.