Yes, Windows 7, 8 or 8.1 installed on a Mac using Boot Camp is eligible for the free upgrade to Windows 10. And, no, Apple doesn't officially support it yet, but that doesn't mean you can't install Windows 10 and use all of the available Boot Camp features like the trackpad, keyboard illumination and screen brightness control.
Apple's Boot Camp installer isn't up to speed yet but you can download Windows 7/8/8.1 drivers (which will work with Windows 10) and install them individually depending on your supported Mac:
Earlier supported Macs:
https://support.apple.com/kb/DL1720?locale=en_US
More recent Macs:
https://support.apple.com/kb/DL1721?locale=en_US
After un-zipping, open the BootCamp5.1.5621 or BootCamp5.1.5640 folder, then open BootCamp>Drivers.
Each folder contains the setup programs for installing the Apple-specific drivers. If you're unsure of your Mac's configuration, run each one. The setup program will check and let you know if it's not right for your Mac. Also, if your Mac has an AMD/ATI or Nvidia graphics card, Windows 10 will probably install the Windows 10 gaming driver. If that's the case, don't bother with the Apple-supplied graphics driver.
It'll do until Apple announces official Windows 10 support.