I have tried now a workaround by using bootmgr to load the VHD as FILEDISK .
Windows Boot Manager
--------------------
identifier {bootmgr}
description Boot Manager
locale en_US
nointegritychecks Yes
displayorder {a80a400f-f3d3-4c07-b7b9-8729c6f78b37}
{7eca6a46-2bb5-4d8c-b556-40d70ec55e93}
bootsequence {bootmgr}
timeout 30
Windows Boot Loader
-------------------
identifier {a80a400f-f3d3-4c07-b7b9-8729c6f78b37}
device vhd=[locate]\RADIO.VHD
path \Windows\system32\winload.exe
description Boot from VHD
locale en-US
testsigning Yes
osdevice vhd=[locate]\RADIO.VHD
systemroot \Windows
detecthal Yes
Real-mode Boot Sector
---------------------
identifier {7eca6a46-2bb5-4d8c-b556-40d70ec55e93}
device boot
path \grldr.mbr
description Realmode App Loader
So de MBR of the usbstick is set to bootmgr and the /boot/bcd has two entries:
1. pointing to the VHD with test-signing enabled.
2. pointing to grldr.mbr which loads menu.lst which has two entries, one for loading VHD as FILEDISK the other as RAMDISK.
RAMDISK works fine,
FILEDISK failed with INACCESSABLE_BOOT_DEVICE
The VHD has also a /boot/bcd inside.
H:\Boot>bcdedit /store h:\boot\bcd /enum
Windows Boot Manager
--------------------
identifier {bootmgr}
device unknown
description Windows Boot Manager
locale en-us
inherit {globalsettings}
default {default}
resumeobject {1bdf8bc8-0f0a-11e9-9a75-d05099614b6b}
displayorder {default}
toolsdisplayorder {memdiag}
timeout 30
Windows Boot Loader
-------------------
identifier {default}
device unknown
path \windows\system32\winload.exe
description Windows 8.1
locale en-us
inherit {bootloadersettings}
testsigning Yes
allowedinmemorysettings 0x15000075
osdevice unknown
systemroot \windows
resumeobject {1bdf8bc8-0f0a-11e9-9a75-d05099614b6b}
nx OptIn
bootmenupolicy Legacy
detecthal Yes