I have a Montage with faulty main output jacks.

You should get that fixed as soon as possible.

I need to change all my performances to be output thru the assignable outs.

Assigning all of your Performances so that they use the assignable output may sound like a good idea but it is NOT. The Assignable L&R are quite different from the Main L&R Outputs. The Main L&R Outputs include the System Effects (Reverb and Variation), it includes the Master Effect, and it also includes the Master EQ... none of those are available via the Assignable L&R Outputs. Again, get your "faulty main output jacks" fixed as soon as you can, so that you have a fully working instrument.

I have Melas' montage tools but for the life of me I can't find the output assign parameters in this software. Am I missing something? I was also wondering if there's a global output assign function so I don't have to change each and every one of my performances.

As a general rule, you ought to be learning where the parameter assignment for the Assignable Outputs are made in the MONTAGE, then you would know exactly where to look in the Melas Tools.

In order to assign a Performance to the Assignable L&R Outputs, you would need to edit each and every PART of each and every Performance and route the PART to the "AsgnL&R", then STORE a new version of the Performance.
For example, let's take "CFX Concert"
From the HOME screen
Press [EDIT]
Press [PART SELECT 1]
Touch "Part Settings" > "General" > find the PART OUTPUT parameter set it to "AsgnL&R"

Press [PART SELECT 2]
Set it to "AsgnL&R"
Repeat for PART 3 and PART 4
Press [STORE] to write your edited version of the Performance to the USER BANK.

Rinse and repeat 2655 times for each of your MONTAGE factory Performances (yikes!)
If you are looking for the parameter in the Melas Editors then now you know it is a per PART parameter called PART OUTPUT.

Alternate Method
Although I had not thought about this (until your question)... it occurs to me there is a workaround, until you get your instrument fixed...
You can setup a QUICK SETUP template to change all the active KBD CTRL PARTs to "PART OUTPUT" = "AsgnL&R"
The way this will work is you would need to recall a PERFORMANCE, then activate the QUICK SETUP template (each time).

The QUICK SETUP template works to make the stored changes when you recall it. It does NOT remain between different Performances - in other words, as soon as you move to the next program the instrument reverts to normal (which means everything returns to going out the Main L&R). You can reprogram Template #3 quickly enough...

Here's how to setup this workaround:
Press [CATEGORY SEARCH]
touch "Init" > select "Multi/GM"
Press [ENTER]

Press EDIT]
Press [PART SELECT 1]
Touch "Part Settings" > "General" > set the each of the first 8 PARTs so that OUTPUT SELECT = "AsgnL&R"

Press [SHIFT] + [UTILITY]
Touch the box in the lower right corner that says "Store Current Settings"
Set the "Setup No." = 3
Rename the Setup: Audio to AsgnL&R
touch "STORE"

Now whenever you want to route all PARTS to the AsgnL&R Output you just need to recall this Quick Setup:
[SHIFT] + [UTILITY] to go to Quick Setup
Touch "3" to recall your settings.