Unfortunately, MapleStory is not compatible with Windows 8. I can't be sure why but I can say for certain that it will not work on the Windows 8 Developer Preview. However, MapleStory did not work in Windows 7 when it was first released so it should be fine when the final build is released.

Compatibility mode doesn't work for the same reason that Themida is included in the executable. In an attempt to stop users hacking the MapleStory client, the developers have rendered compatibility mode useless.

Any luck yet on the subject?

If trying to use the Program Compatibility Troubleshooter from Control Panel, I end with this error message, originating from the MapleStory.exe (the error windows is active and the MapleStory.exe mushroom icon is present on the taskbar)

Successfully started it. Even though MapleStory includes a security feature that blocks it from running itself inside a virtual machine, that feature does not being enforced, when running Windows 7, booted from a VHD drive on the physical machine (Physical VHD Boot). That can count a sort of a secondary OS installation, but without partitioning the physical drive.

Code:

Windows PowerShell
Copyright (C) 2011 Microsoft Corporation. All rights reserved.

PS C:\Windows\system32> bcdedit /enum
Windows Boot Manager
--------------------
identifier              {bootmgr}
device                  partition=\Device\HarddiskVolume1
description             Windows Boot Manager
locale                  en-US
inherit                 {globalsettings}
integrityservices       Enable
default                 {current}
resumeobject            {b01ee452-0530-11e1-a532-bf8f9db8a857}
displayorder            {current}
                        {b01ee45d-0530-11e1-a532-bf8f9db8a857}
toolsdisplayorder       {memdiag}
timeout                 30
displaybootmenu         Yes
custom:26000025         No

Windows Boot Loader-------------------
identifier              {current}
device                  partition=C:
path                    \Windows\system32\winload.exe
description             Windows Developer Preview
locale                  en-US
inherit                 {bootloadersettings}
recoverysequence        {b01ee454-0530-11e1-a532-bf8f9db8a857}
integrityservices       Enable
recoveryenabled         Yes
osdevice                partition=C:systemroot              \Windows
resumeobject            {b01ee452-0530-11e1-a532-bf8f9db8a857}
nx                      OptIn
custom:250000c2         1
hypervisorlaunchtype    Auto

Windows Boot Loader
-------------------
identifier              {b01ee45d-0530-11e1-a532-bf8f9db8a857}
device                  vhd=[C:]\VHD\win7.vhd
path                    \Windows\system32\winload.exe
description             Windows 7 (VHD)
locale                  en-US
recoverysequence        {b01ee45e-0530-11e1-a532-bf8f9db8a857}
recoveryenabled         Yes
osdevice                vhd=[C:]\VHD\win7.vhd
systemroot              \Windows
resumeobject            {2334aacf-2c9c-11e1-bdb7-806e6f6e6963}
PS C:\Windows\system32>

But that is not an inside-Windows 8 start though. Better than nothing.