- Select files to be 'touched' in the Finder.
- Press Command-Option-I to open the multiple item info inspector window (File » Show Inspector).
- Toggle/activate the Open With pop-up menu. You needn't select anything, just click to activate the menu.
[robg adds: In my testing, I didn't have any luck with the time changing just by merely activating the menu; I had to select the currently-active option for the chosen files. If I selected files with different Open With settings (such that the pop-up menu reads Not Applicable), I couldn't change the modification timestamp at all via this method.]

