I just got my minipov3 built and I am going to be playing with the software a bit in the next week. I'll throw together a version with multiple distinct arrays for you when I get to that.
Max size with the normal firmware should be around 1800 columns, it looks like there is about 228 bytes of code plus one byte per column, using all_leds as a baseline.
For longer messages one idea would b to move to a pre-programmed character set. Say you have an alphabet of 64 characters (a-z,A-Z,0-9,space,period), and each character is 6x8, then that takes up 384 bytes of storage, and you can store your message in the remaining space (minus a bit more code) at 6x the density of the default method, call it 1300 characters (7800 columns).