Inspired by Mac OS X’s Dvorak-QWERTY Command layout, I have found ways to convert my Dvorak layout to QWERTY only while a modifier key is held down on Windows, Mac, and Linux for both the Dvorak Simplified and Programmer Dvorak layouts.
Mac OS X
I came up with a Programmer Dvorak–QWERTY Command setup that I use daily on my MacBook. It is effectively QWERTY when Control, Option, or Command (⌘) are not held down.
On Windows I used AutoHotkey to rebind my keys when Control, Alt, or Super (Windows) are not held down.
Back when I used desktop Linux, I used dvorak-qwerty to swap my keyboard layout. I found this to be the least reliable layout between the three operating systems, but I never found a better way than this program which rebinds at the X11 level.