The simple answer is that there has been absolutely no way to do this since Windows Vista. There is some hope, however - with the new Microsoft's increasing emphasis on open source, there's a good chance that a significant enough amount of (Windows 10) users requesting the feature may be enough for them to finally introduce this as a feature. There's even an app bundled with Windows 10 for this express purpose: the Feedback Hub.
If, like me, you're coming across this answer and you really want the long-overdue capability of removing the single-file limit for the "Open With" context menu item, in the same way that it's currently possible to remove the 15-file limit for the "Open" and "Edit" content menu items, then you can make a request over at Windows 10's Feedback Hub. Currently, that is everyone's best chance at seeing this feature become a reality.
