I have a matsui pf-a701w
Custom firmware? No, nobody has designed a custom firmware for that device and I don't think anyone is working on it (because honestly the device is cheap junk). There aren't many digital-photo-frames that have been altered in that way.
You could however, do any of the following with minimal effort.
1. Download a book from the guttenburg project, print it as a series of images, and copy those to your photo frame. Instant book-reader.
2. Do the same as number 1, but make images of a bunch of recipies. Digital cookbook!
3. Glue a 3"x5" print to the screen... Enjoy better resolution!
Oh.. Ok... I did get it from a car boot sale... LOL