It is indeed a nice addition. Other fundamental factors that I think are a good addition are an increase in inventory (to assets), three year total asset variability and any type of factor that checks for profitability or cashflow generative ability in the past few years.
Actually, I never really thought of this before. I guess this also means that there are factors that add (negative) returns to the bottom bucket in a Long system, but that do not add positive returns to a top bucket in a Short system. That’s actually quite confusing. I think in the future I will have to run it in both ways.
Besides this threat, I found these resources to be handy:
- Portfolio hedging/shorting with options - #2 by yuvaltaylor (Yuval’s post)
- My Top Ten Factors-for Going Long and for Going Short - Portfolio123 Blog (P123 blog)
- Short Selling Techniques To Boost Returns And Lower Crash Risk | Seeking Alpha (Hemmerling’s post on Seeking Alpha, as mentioned above)
I’ve created a simple (equal weighted) public ranking system based on the factors listed there and added the factor mentioned by InmanRoshi (sgandagr%pyq), as well as the inventory increase factor, the asset variability factor and a cashflow generative ability factor I mentioned myself.
The ranking system can be found here: https://www.portfolio123.com/app/ranking-system/438228.
In this case the Easy to Trade US Long system generates about 14% return for the ‘worst’ bucket. In case of a Short system it is about zero.
I’m hoping this will lower the bar for others to share some more factor ideas for going short 
!
Victor