I recently discovered that the HFS filesystem is quite easy to boot to from grub4dos, just by using chainloader.
You can make an image of an HFS partition using RMPrepUSB - Drive -> File.
To see how you can make a Yosemite OS X installer image which you can boot to via grub4dos, see my blog post here.
A simplified menu is:
title OS X Installer\n Boot from a partition image\n This uses partition 3 of the USB drive set HFS=/OSX_Installer.HFSPTN set E2BDEV=hd0 #enable parttype output debug 1 parttype (%E2BDEV%,2) | set check= debug off set check=%check:~-5,4% # make empty table entry in 3rd position in ptn table if "%check%"=="0x00" partnew (%E2BDEV%,2) 0 0 0 echo if /i "%check%"=="0xAF" echo WARNING: MBR PARTITION 3 IS IN USE (Type AF = HFS partition) set ask= if /i "%check%"=="0xAF" echo && set /p ask=OK to delete the HFS partition 3 IN MBR? (Y/[N]) : echo echo # if answer Y, make empty table entry in 3rd position in ptn table if /i "%ask%"=="Y" partnew (%E2BDEV%,2) 0 0 0 debug 1 parttype (%E2BDEV%,2) | set check= debug off set check=%check:~-5,4% if not "%check%"=="0x00" echo ERROR: MBR PARTITION 3 (%check%) IS ALREADY IN USE - cannot continue! && pause && configfile /menu.lst errorcheck off if "%check%"=="0x00" partnew (%E2BDEV%,2) 0xAF %HFS% || echo && echo -e ERROR: %HFS% is not contiguous or is BAD!\nPlease fix using RMPrepUSB-CTRL+F2. && pause Press a key... && configfile /menu.lst rootnoverify (%E2BDEV%,2) > nul chainloader (%E2BDEV%,2)+1 > nul boot