At long last I've discovered an easy way to do this. Not by using a Microsoft multi-boot but by using an independent multi-boot. The OS are "hidden" from each other by removing their drive letter in Disk Management. I've been using TeraByte scripts to restore the EFI partition of the desired OS. The scripts can be on a UFD or on the HD (bootfile method).

My current HD has Win7, two Win8, Win10 and Ubuntu.

Instructions for a four OS install...

There were 4 entire drive images. Win8, Win7, Win10  and Ubuntu. Ubuntu had grub installed in the EFI, not the First Track. Win7 and Win10 had originally been MBR systems. Both had been converted to GPT which explains the lesser number of partitions.

Starting with a Win8.1 system. The partitions were...

Recovery
EFI
MSR
Win8.1

The following steps were performed in IFL.

The EFI partition was imaged to create this file... efi8.tbi

Next, the Win10 image was used.
The EFI image was restored to the EFI partition on the HD.
The Win10 image was restored to Free Space following the Win8.1 partition. The "Update Boot Partition" option was used.
The EFI partition was imaged to create this file... efi10.tbi

Next, the Win7 image was used.
The EFI image was restored to the EFI partition on the HD.
The Win7 image was restored to Free Space following the Win10 partition. The "Update Boot Partition" option was used.
The EFI partition was imaged to create this file... efi7.tbi

Next, the Ubuntu image was used.
The EFI image was restored to the EFI partition on the HD.
The Linux Native image was restored to Free Space following the Win7 partition.
The Linux Swap image was restored to Free Space following the above partition. The "Update Boot Partition" option was used.
The EFI partition was imaged to create this file... efiUB.tbi

Scripts can be used for the following.

efi10.tbi was restored to the EFI partition and computer restarted into Win10. Win8.1 and Win7 drive letters were removed in Disk Management.
efi7.tbi was restored to the EFI partition and computer restarted into Win7. Win10 drive letter was removed in Disk Management.
efi8.tbi was restored to the EFI partition and computer restarted into Win8. No drive letters needed to be removed in Disk Management.
efiUB.tbi was restored to the EFI partition and computer restarted into Ubuntu.

Finished.

Even better than removing drive letters. Change the Partition Type of the non booting Windows partitions to a type the Windows file system can't access. Such as Linux Data.


Code:

sub main()   ext("SET PART TYPE 0 0x06 {EBD0A0A2-B9E5-4433-87C0-68B6B72699C7}")   ext("SET PART TYPE 0 0x04 {0FC63DAF-8483-4772-8E79-3D69D8477DE4}")   exec("/tbu/imagel --r --uy --um --d:{FAF22757-A01E-4689-B1E8-330A2AFB0EA2}@0x2 --f:#0x64DB89F3@0x1:^"/Images/NewMB/multi/used/e8_2^" --rb:4 --nvn:^"^"", 1) end sub