The Earthcomber Updater doesn't directly support Missing Sync. However, it can be configured to work with Missing Sync.

Setting Up Earthcomber Updater to Work with Missing Sync
Basically, you need to configure the Updater to just download the files to a specific directory, typically your Missing Sync Install directory. Here's how:

1. Start Earthcomber Updater, and type Alt-L. You get the switch screen. (Note: On a Mac it may be "Option-L" or "Control-L".
2. From the drop-down list choose PALMBASIC. Click OK.
3. The Earthcomber Updater will restart itself.

Updater will now place files in a directory of your choosing. You need to choose a directory for this, do it now by clicking the "Choose Directory" button. Your install directory is typically something like:
Documents/Palm/Users/Treo 700p/Files to Install
However, the "Treo 700p" should be replaced by the name you've given your device.

The above process works well. However, each time you sync, the files get installed to your device and removed from your "Files to Install" directory. This makes the Earthcomber Updater think you need to update those files every time it checks for updates.

Helping Earthcomber Updater Correctly Identify When an Update Is Needed.
A user, "Tom", provided the following solution:
Just thought I'd let you know how I resolved this issue. I wrote a shell script which runs the updater, saving updates in its own special place, and then copies any new files into the Palm install folder. The script could be cleaned up, and it might be possible to do this with Automator or AppleScript, but this does the job for now.

Use or discard as you wish. It will have to be modified for some directory paths, at least. Here's the script (don't include the "CUT HERE" lines):

---- CUT HERE ----8<-------- CUT HERE ----8<-------- CUT HERE ----8<----
#!/bin/sh
#
#
# Initialize variables.
#
ECPATH="/Volumes/Other/Apps"
ECFILE="$ECPATH/Earthcomber Updater.app/Contents/MacOS/JavaApplicationStub"
ECUDAT="$HOME/Library/Application Support/Earthcomber Updater/Updates"
# NOTE: You must change "Treo 700p" below if your device's name differs.
PALMDT="$HOME/Documents/Palm/Users/Treo 700p/Files to Install"
STAMP="$HOME/now"
#
#
# Timestamp.
#
touch "$STAMP"
#
#
# Run the updater and wait for it to exit.
#
"$ECFILE"
#
#
# Copy any new files to the Palm install folder.
#
find "$ECUDAT" -type f -newer "$STAMP" -print0 | xargs -0 -J % cp "%" "$PALMDT"
#
#
# Delete the timestamp.
#
rm "$STAMP"
---- CUT HERE ----8<-------- CUT HERE ----8<-------- CUT HERE ----8<----


To run the script, save it to a file and then do "sh filename" in a terminal window. Terminal can be found in the Utilities folder in Applications. Alternatively, you can create an AppleScript application to invoke the shell script. The AppleScript can be something like this:

do shell script "$HOME/ecu"

where "ecu" is the name of the shell script, residing in my home directory.