How can I map the Windows key to option in Microsoft Remote Desktop version 8?

Indeed, this appears to no longer be possible without 3^rd-party software.

Adding the same preference key to the new app seemed to not make any difference - it appears that this ability has been removed from the app.

As you mentioned, KeyRemap4Macbook would work with the private.xml that you linked to.

Here's the private.xml I ended up using in KeyRemap4Macbook to remap the keys. In particular note that I set it up so that the physical command-tab combination still activates app switching just like in the old version of RDP. Note that you must also enable the existing "Shift_L to Lazy_Shift_L" and "Shift_R to Lazy_Shift_R" options to enable using command-shift-tab to navigate backwards while switching apps.

<appdef>
    <appname>MRD</appname>
    <equal>com.microsoft.rdc.mac</equal> 
</appdef>
<item>
    <name>Swap command and option in Remote Desktop</name>
    <identifier>private.remotedesktopswapcommandandcontrol</identifier>
    <only>MRD</only>
    <autogen>__KeyToKey__ KeyCode::TAB, MODIFIERFLAG_EITHER_LEFT_OR_RIGHT_OPTION, KeyCode::TAB, ModifierFlag::COMMAND_L</autogen>
    <autogen>__KeyToKey__ KeyCode::OPTION_L, KeyCode::COMMAND_L</autogen>
    <autogen>__KeyToKey__ KeyCode::COMMAND_L, KeyCode::OPTION_L</autogen>
    <autogen>__KeyToKey__ KeyCode::OPTION_R, KeyCode::COMMAND_R</autogen>
    <autogen>__KeyToKey__ KeyCode::COMMAND_R, KeyCode::OPTION_R</autogen>
</item>