Although you can't do it directly, I thought of a workaround.
Edit your project to completion with effects, multiple layers and clips
Right & Enable Ripple All Tracks
In Windows Explorer or Finder, make a copy of the .mlt project file
Click at the start of a section & hit Shift+I to trim all tracks & make the current position the inpoint
Click at the end of your section & hit Shift+O to set the outpoint
Export your section project
Ctrl+Z twice to undo your trimming and you should be back to the original state of the project
I know it's now elegant, but it's not that bad and you always have the backup project file if you accidentally save over the version that only has a potion or the entire project