I even tried a plain Windows 10 Installer created in Windows with none of the Boot Camp stuff, on a USB 2 drive and it didn't work. I want to stress that making and using the bootable USB drive is not the problem, I was in fact able to install Windows 10 on an unsupported 2010 WMB with no problem. The problem starts after Windows 10 does it's initial boot from the SSD, after the USB is no longer needed.
There is no Bootcamp.xml file on the BCA created drive, there is the AutoUnattend.xml which I will paste here:
<?xml version="1.0" encoding="utf-8"?>
<unattend xmlns="urn:schemas-microsoft-com:unattend">
<settings pass="windowsPE">
<component name="Microsoft-Windows-Setup" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<UserData>
<ProductKey>
<WillShowUI>Always</WillShowUI>
</ProductKey>
</UserData>
</component>
<component name="Microsoft-Windows-PnpCustomizationsWinPE" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<DriverPaths>
<PathAndCredentials wcm:keyValue="1" wcm:action="add">
<Path>$WinPEDriver$</Path>
</PathAndCredentials>
</DriverPaths>
</component>
</settings>
<settings pass="specialize">
<component name="Microsoft-Windows-Deployment" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<RunSynchronous>
<RunSynchronousCommand wcm:action="add">
<Order>1</Order>
<Path>cmd /c "FOR %i IN (X F E D C) DO (FOR /F "tokens=6" %t in ('vol %i:') do (IF /I %t NEQ "" (IF EXIST %i:\BootCamp\BootCamp.xml Reg ADD "HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Environment" /v AppsRoot /t REG_SZ /d %i /f )))"</Path>
</RunSynchronousCommand>
</RunSynchronous>
</component>
</settings>
<settings pass="oobeSystem">
<component name="Microsoft-Windows-Shell-Setup" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<FirstLogonCommands>
<SynchronousCommand wcm:action="add">
<Description>BootCamp setup</Description>
<CommandLine>%AppsRoot%:\BootCamp\setup.exe</CommandLine>
<Order>1</Order>
<RequiresUserInput>false</RequiresUserInput>
</SynchronousCommand>
</FirstLogonCommands>
</component>
</settings>
</unattend>