I've emailed Favero about this, but this reddit thread features a fairly recent answer from their helpdesk (11 days ago when I initially wrote this).
“As far as the Look version is concerned, we confirm you that Assioma UNO/DUO is used by many customers with pedal extenders up to 25mm without any issue. Talking with our technicians about this topic, they suggested to us that there are some extenders with the hole which could be the most suitable for the installation on the pedal (we remind you that you should not apply any force on the sensor to avoid damages).
Other comments there state that they haven't perceived any significant change in power. This doesn't rule out that it has a theoretical effect because it extends the lever on the spindle but I'm not enough of a physicist or mechanical engineer to understand what this could mean to power readings via strain gauges.
Here's what I got as response
Even if we are aware that some cyclists use pedal extenders, since we have not tested them directly in our facilities, I kindly inform you that we cannot guarantee their full compatibility and safety of use with Assioma. For this reason, I do not recommend using pedal extenders also because any possible damage to the product due to its incorrect use, may not be covered by the warranty.
I kept asking about impact on accuracy of their powermeters with extenders and got this:
As mentioned in my previous email, as far as I know, Favero Electronics never tested pedal extenders in our facilities but I confirm that we are aware that some customers use them, in particular the ones with the hole. Since they have never been assessed in our facilities, unfortunately, neither I nor our engineers can effectively confirm if the employment of pedal extenders could have any influence on the accuracy and/or display of the power readings.
To sum it up
- It works for some riders based on user experience.
- Favero as one of the well-known manufacturers of pedal acknowledges that these combinations work for some riders out there but haven't conducted any testing and would potentially deny warranty claims related to pedal damage when using extenders.
Verdict: Use at your own risk.