Hi all, thought I would share how I got Mega-Em working consistently on vanilla Dosbox SVN.
This basically provides me with support for the Ultrasound, Sound Canvas, and MT-32 options for various games without having to rely on patches. Mega-Em version 3.11 is nice because it will turn emulation on and off as needed, i.e. if you have SC-55 emulation running and choose a game configured for Gravis Ultrasound, Mega-Em will unload itself without any input. It will also re-load when you exit the game.

This guide assumes that you know how to use DOSBox and configure normal Gravis Ultrasound support. With that being said...

---FILES NEEDED---
DOSBox Vanilla SVN (I tested on r4007)
Huggybaby's Ultrasound Directory (I used the 4.11 version instead of the Pro Patches; I don't know if this is a factor)
EMM386.EXE (I used the Microsoft-approved version included on the GUS PnP installation disk)
A way to load device drivers from the command line (I used DDLOADER.COM sourced from https://www.hiren.info/download/dos-files/ddloader.com )
Mega-Em 3.11 (from the GUS PnP disk) with the necessary bank file (LARGE.BNK; sourced from Mega-Em 3.06b, but I can't remember the download site)
I have attached a zip file with the last three items for convenience. If these are not ok to distribute, I apologize.

---DOSBOX.CONF CHANGES---
[midi]
mpu401=none (to ensure that you are properly emulating Sound Canvas/GM)
mididevice=none (to ensure that you are properly emulating Sound Canvas/GM)
[dos]
ems=false (EMM386 WILL NOT WORK IF THIS IS SET TO ANY OTHER VALUE)

---SETUP AND USE---
1. From the zip provided, place DDLOADER.COM and EMM386.EXE in a convenient location. I chose the root of my mounted folder.

2. Place the remaining files in your Ultrasound folder (for me, the default c:\ultrasnd). Allow files to be overwritten.

3. Start DOSBox and load EMM386.EXE. You will need to use the following syntax:
DDLOADER.COM EMM386.EXE M#
Substitute # for a number from 1-9. This will set the page frame base address to a location from C000h-E000h in memory. If successful, it should look like this:

If you set the number too high (10-14 will place the page frame in the 8000h-9000h range instead), or if you enter the wrong option, you will get the following error:

The only solution for this that I am aware of is to restart DOSBox and choose the correct range. I don't know how to unload EMM386.EXE otherwise. --CORRECTION: IF YOU USE THE CONF -SET EMS=TRUE COMMAND, IT WILL UNLOAD MEGAEM AND EMM386--

4. Next, navigate to your Ultrasound directory and run MEGAEM.EXE. You should see an initial configuration screen like the one pictured below.

5. Select Ok and then Change Configuration. I recommend the settings show below. Sound Blaster emulation doesn't make much sense with DOSBox's robust support for it, and NMI won't work even if enabled. Interactive Mode allows you to use this menu-based system to change settings in the future via MESETUP.EXE (also in the Ultrasound directory)

6. Press F10 to save the changes you've made and then "Run Mega-Em with current settings". If it works, you should see output similar to the below picture:

7. Test with various games and settings. I tested Gravis and Sound Canvas support with Hocus Pocus, and MT-32 with Police Quest II, and they worked reliably for me.

---CAVEATS---
-Although I was able to reproduce the conditions to get Mega-Em working, I have only used it with an extremely small selection of games. Obviously, your mileage may vary.

-Mega-Em will not work under Windows 3.1 at all. You should unload Mega-Em (with MEGAEM.EXE -U), and turn off EMM386.EXE (EMM386.EXE OFF) before starting Windows. The normal drivers and functions can be used, though.

-Mega-Em can only support either the MT-32 or SC-55, but not both at the same time. You will have to reconfigure and unload/reload Mega-Em to switch emulation.