I found a workaround based on something similar from Kerbal Space Program's 64 bit version back when they had a 64 bit mode.
"Method 2:
1. In the Steam games list, R-click PBA: Properties -> General -> Set Launch Options...
2. type in the following:"the path of PinballArcade11.exe" %command%
example: "C:\Program Files (x86)\Steam\steamapps\common\PinballArcade\PinballArcade11.exe" %command%
3. run PBA as usual"
From: http://steamcommunity.com/app/220200/discussions/0/35219681790813176/#c35219681792617623
This sets DX11 as default from a steam://runappid shortcut and doubleclicking on either exe in the folder even from my testing.
Hope this helps,