Is there any way to change the font SYSTEM WIDE on MacBook Pro running OSX Yosemite? How can one change the system font system wide in OSX Yosemite? I am using Tinkertool at present but it only changes font in applications and not the Finder, status bar or any other fonts!
As of today the only 2 ways i know of are:  


Hack the CFF headers of a font: github example and put the modified font(s) in a prioritized font folder.
The other one indeed is TinkerTool. However you have to spoof your version of Mac OS X temporarily to make it work. According to this site and this post 4th entry:
Go to System/Library/CoreServices inside your OS X hard drive and open the file SystemVersion.plist with TextEdit. Change the system version listed under "ProductVersion" to 10.9. You should also change "ProductUserVisibleVersion", just so that you can confirm that the edit worked later by going to the About This Mac page.