Thanks for your interest in expanding the range of ways Ubuntu can be used.
Your argument for this configuration appears to be that the resulting install can be reused portably as a VM image, as well as being booted directly on the hardware. However, this is not truly the case: to be booted as a portable VM image the bootloader must be set up within the loopback image, and to be booted on the hardware the bootloader must be set up /outside/ the loopback image, on a boot partition.
This dual bootloader setup would be fragile, and impossible for us to support. Added to this the fact that a loopback disk will have worse disk I/O due to the added filesystem layer, I do not think this is an option we could reasonably include in the installer. Our recommendation is still to use the installer’s repartitioning support, to install Ubuntu alongside Windows instead of loop mounted from within it.