If I get time tomorrow, I will try to complete an install of all of the packages, but as a point of departure, everything else looks good.

BTW, not crazy about LXDE-QT, but I really appreciate the ability to choose login managers during the expert install

I have used grub2 with uefi in a multiboot situation, and it's pretty easy once you see what's going on. Each OS will add a directory with a bootloader to the efi partition. You can see the order of the bootloaders by running 'efibootmgr'. Whichever one is first in the boot order will be used to boot the system. Whichever one you installed last will be the first in the boot order.

If you have more than one linux system, boot the first one and add the second linux to the boot menu. (Run update-grub in debian-based systems.) If you let devuan be in charge of booting, you can boot slackware from devuan's grub menu just like you'd boot windows from the grub menu. No need to use elilo.

There might be a way to use elilo and let devuan boot from the slackware menu, but I don't know elilo at all.

If there's a kernel upgrade, you may need to update the boot menu. This will happen automatically in devuan. If devuan is managing the boot, and you have a kernel upgrade in slackware, you may need to run update-grub in devuan again. (unless your grub entry for slackware uses a constant name for the kernel and initrd, such as /vmlinuz and /initrd.img)

The general recommendataion against using outside repos is because they haven't been tested with devuan. If that repo contains programs or libraries that require versions that aren't in devuan, you could end up with a big mess of dependency problems. Since you might be the first one trying this, take good notes and let us know how it goes.

Here's a good reference for uefi bootloaders (including rEFInd, which is another possibility.)
http://www.rodsbooks.com/efi-bootloaders/index.html

I am planning the install a fresh Devuan system with the KXStudio repo included for audio production.

The install will be a triple boot (Win7, Devuan and Slackware), so I am interested in getting any feedback possible on extlinux in this connection. Win7 is already installed in UEFI mode and I am wondering how extlinux deals with (or the user) deals with changes to the system, such as kernel updates. Slackware64 14.2 uses elilo, which works with UEFI, but not in a multi-boot scenario. I find GRUB2 to be quite frustrating and would certainly consider using extlinux for a Devuan install, even without a multi-boot scenario, but I am a bit fuzzy on the details in reference to system upgrades, and I haven't been able to find any documentation on that particular aspect of extlinux.