I've changed my mind... I will be using a SD card for animations.
It will also be used to store the highscore etc. The built in EEPROM has a "very" limited write/erase cycle so I figure I'd keep the internal memorycards intact and store everything on external cards to minimize the risk of my pinball machine breaking down during a tournament or whatnot.
The Chipkit 512 KB flash storage means around 120 frames of animation. That data is probably best used as common animation that needs to be readily available and the SD-card ones will be mode-specific graphics. With SPI the datarate is fast enough, and I figure I could use the natural pauses in the game to load new animations into RAM. The current game logic was actually already prepared for this so it would mean minimal changes and/or interruptions in gameplay.
I've also got basic serial communication between master (Arduino) and slave (Chipkit) going on, so the host can send "playMusic", "playAnimation" etc commands to the slave which will then perform them. I will probably do at least one rewrite of that library as time progress to further streamline the communication, but at the moment it works fine. It is prepared for (and is currently) sending batch jobs of commands, but it's uncertain if the final version will send single commands right away or batch them together for sending at the end of the frame. There are different benefits for both versions.
I've also refactored the code to work with two different boards:
Arduino - Switches, Solenoids, Game logic and SoundFX.
Chipkit - Display, Lights, SD card and Music
With the transition to the Chipkit, it unfortunately turned out that (most likely) the voltage difference from 5V to 3.3V does not allow the display to properly function. I've been in touch with the manufacturers of both the display and the Chipkit and we've come up with the same conclusion. So, I've ordered a 3.3->5V logic translator chip now which will hopefully solve the problem. Kind of a bummer, but it will be worth the extra effort for all the sweet improvements it offers.
The game logic ... Ah, the fun part!
The ruleset is shaping up quite nicely and as it seems to me - a nice adaptation of the original concept. When reading the rules I'm quite satisfied that the gameplay will be fun as well. I guess the hardest part will be actually finding sound and graphics for the game, but if all else fails, I will do them myself.