Yes, custom fonts are easily added to any ereader of the Kobo family. An article on MobileRead fully explain how.
Fonts can be be very easily added to the Kobo family of eReaders. With
the Kobo attached to your computer, create a new directory called
fonts in the root directory of the device (the same location as the
.kobo, .adobe-digital-editions ...
It depends on specific device and its architecture.
Those that are built on top of Android, typically tend to get AOSP ports, e.g. Nook (here's Simple Touch instructions), or Kindles - the latter even has CyanogenMod ROM.
Sony PRST 1 was hacked to expose full underlying rooted Android but I'm not sure if AOSP based ROM is available for it - this might be ...
Kobo ereaders are known for their font customization capabilities. They have a dozen of preinstalled fonts, and a couple of them aimed exactly to dyslexic readers.
Besides selecting the font, you can also tweak it to your needs with many options for dimensions, spacing, margins, justification and thickness.
You can read a couple of reviews, here and here, ...
Apologies for replying as an answer, I have no sufficient reputation to comment.
I am the creator of Penelope. I thought about this for a bit, but the main problem is that I am continuously asked to "convert" materials that are clearly pirated from copyrighted, commercial dictionaries --- which I always refuse --- and hosting them onto a public web site ...
KOReader is an ereading software available for several devices and platforms. Quoted from Mobileread wiki:
KOReader is a document viewer for Linux based E Ink devices. Currently
there are versions for Kindle, Kobo, PocketBook, and Android devices.
The installation procedure depends on the device. Support forums are available on Mobileread.