I finally had the time to test on the RPi. I found a way to make it work, using ASTAP, but it hasn't been easy for a couple of reasons.
First, let's address the Use Scale and Use Positions switches: they don't work. The scale and position gets passed on to the solver, no matter what these switches are set to. At least, using ASTAP. The only way to really get a blind solve is to strip those info from the fit file itself.
Now, having done exactly that, I tested and confirmed that ASTAP can blind solve, and it's quite fast too. The most problematic image took 40 seconds on the RPi, which I think is fantastic. But another problem bugged me for a while. It seemed like Ekos liked to pass the "-r 15" parameter to ASTAP no matter if I changed the max radius search option. This was because I was changing another profile, not the one I was actually using. I think that part of the Ekos config could use some work, it's easy to get lost.
But finally I have a solution that can work on the RPi for blind solving images.
I'm still going to try and put up a nova like server, with the help of rlancaste I hope to be able to succeed. I also hope this thread is going to help somebody with problematic images in the future.
BTW: is there any specific reason the default search radius is so small? Upping it to 180 doesn't slow down anything if coordinates are close enough, just a thought.