I have the same bags and would stitch it with safety wire on all four sides if accessible inside the bag (didn’t look at mine). Then, would pop rivet some home made, thin aluminum brackets on three sides to the main case and the broken tab. Seal it up with silicone and ride.

 

If you can get to the inside to reseat the parts,some fibreglass would maybe do it.
Fibreglass polyester resin will not chemically bond to those types of plastic, but key it hard with 40 grit paper and a good mechanical bond is possible.

 

Drill a series of small holes around break and through hook along edge of break. Stitch together with short pieces of wire twisted together at the ends to tighten. Use black Shoe-Goo to cover seam, holes, wire, and fill inside of hook completely. This stuff will harden like steel and stick like crazy to the plastic case material. It is also completely waterproof. Did this to a Givi E22 case a couple of years back and it is still holding together. Instead of wire, I used zip-ties but the result was successful and has never leaked. Fast fix that has fixed fast.

 

The idea of stitching with tie wire is a good one. I stead of shoe good, I would use CA / supaglue and baking powder in layers. Build it up.

On the inside, I will use 0.8 or 1mm gal steel strip about 300mm wide and then position that and use the CA glue / baking powder bond. Possibly key the area with 40 grit beforehand as mentioned from earlier poster.

I find the properties of CA glue / baking powder highly durable and with decent tensile strength. You would probably crack the adjacent area of the pannier before the patch failed based on experience of fixing hard plastic camera housings with the method.

 

I have the same bags and my plan if I ever break them (and it's a crazy hail Mary plan) is to try to desing the whole affected part in a CAD sofware (that I don't know how to use yet ) and have it 3D printed at a proffesional shop, out of a similar material. And then hopefully being able to reassemble the whole damn thing.

 

This is on eBay plastex stronger then what your fixing comes in black I used this to repair motorcycle body tabs.

 

This is on eBay plastex stronger then what your fixing comes in black I used this to repair motorcycle body tabs. View attachment 332139

You can get this on Amazon as well. I have used this method for literally decades.
Typically, I just buy the glue in a bigger bottle and use common baking soda. You can also addgraphite powder in with the baking soda. The benefit to his kit is the molding clay. You can use it on a good tab to replicate what you need.
Any way, this series of "How To" videos from his website is a must watch.
Fix broken plastic, Plastex

 

A couole of pics showing the “repair” using stitch & Goo
The sidecase was cracked through along the red paint line.
Shoe-Goo has held up for 2 years and has not leaked. On bike, repair is virtually unnoticeable.