8- It's alive!

Well... as planned, the most difficult part of this mod was the mechanical aspect (eh, cabling a ¼" phone jack isn't exactly rocket science...). Here's the last pictures in batch:

7- Drill, baby, drill!

Holes in sheet metal?  Conical step bit!

6- Displaced cap

Cap leads must be soldered on the PCB back (no room in front)

Tucked away under RCA platform

Note the shaded patterns underneath  each capacitor
circle: they identify the negative  lead. 

5- No room for you here

Move aside. Yes, you.

4- A tight fit

3- Electrical wiring

Here's the boards before the modification. Note the wires 2 and 4 (marked W2, W4 as silkscreened on the PCB): they're signal ch.1 & 2 and will be cut. W1 and W3 are signal grounds. The red circle is the capacitor to be displaced.

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The modified boards. The two INSERT jacks are added and the cap desoldered and placed horizontally:

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The detail of the 1/4 jack wiring. This is known as a TRS "tip send ring return" configuration, the standard for most recent mixers for unbalanced mono effect insert point. Dashed line is cut wire W2 above:

There's a 120Ω resistor on the send tip as described in this Mackie mixer schematic I don't know if it's mandatory... Eh I'm a physicist not an engineer!  Thanks to Mike Rivers on Mackie Forums, it's called a build-out resistor and prevents oscillations when cable capacitance is added to signal path.

2- Let's get physical

The physical aspect of the mod: being a compact mixer with tight components integration, finding enough place for two 1/4 jacks is difficult. Only a particular compact jack from Neutrik manufacturer will do and we must displace a component to free some space: a big 1000 uF capacitor (cap in red on the drawing and purple is free space).