I answered already at the MeeGo forum, but perhaps someone over here will find this small guide useful, too.
Sorry for the double posting.

I installed this week also a tripleboot Windows 7, Windows 8 and MeeGo because of a very similar reason. My girlfriend loves to play Plants vs. Zombies and this doesn't support Windows 8. And once played on my Tablet there was no way back to her Desktop.

A triple boot doesn't work using the Windows 8 Boot selector but using the one from Windows 7 it works very well. So you need to install it in a unusual order.

Here are the steps I performed to install the stuff:

1. Windows 8

• Boot from your Windows 8 disk
• perform a manual installation
• clean all aviable partitions during the installation
• create a about 25 GB big NTFS partition
• you get a information that the wizard may create additional partitions to enable all features and a warning that your Installation may won't run without - ignore it
• the wizard created a 350MB NTFS Boot Partition and your 25GB NTFS - delete the 350MB Partition and select the 25GB NTFS for the installation

preparation for Step 2:
Download all needed Windows 7 drivers (especially W-Lan) for your ExoPC - check if your need the 64bit or the 32bit drivers

2. Windows 7

• Boot from your Windows 7 disk
• perform a manual installation
• create a about 25 GB big NTFS partition
• select the 25GB NTFS for the installation
• as soon as you finished install the files from the second NTFS partition you downloaded before the installation - yes, both drives are visible, try to don't get confused - good luck!

3. MeeGo

• Boot from your USB Stick with the latest pinetrail image
• Use the free space for the linux partitions - if you check, you've only wasted 100MB in the beginning because MeeGo will use there 250 for his Boot partition
• reboot, connect a keyboard and go to the terminal
• get root using su and the password meego
• Modify /boot/extlinux/extlinux.conf using vi ("timeout 60", comment out the "menu hidden" line, rename other to Windows (yes, only one at choice) and store the file using [ESC]:wq)

Now you have your tripleboot system ready.
Chose using the sensor in the corner if you want to boot MeeGo or one of your Windows installations. Short press to select, long press to boot. If you select Windows a second Menu will appear where you able to select which dirty evil system you want to boot alternatively.

As ever at your own risk...
Windows may deform your mind so use it carefully and ENJOY THE TRIP!