Rob, actually, I did de-obfuscate that interpreter, only to find the deobfuscated version when I was done, oh well. A sunday well spent. BASIC on the Due sure made my Apple][ days come back to me!
And yes, there are lots of BASIC interpreters to be found, sure one should fit the arduino due pretty well. I might continue on what I have, if it rains a lot this summer.
Stimmer, I did something similar, but looked at it with the scope, seems decently linear.
void setup() {
pinMode(36,OUTPUT); pinMode(37,OUTPUT);
pinMode(38,OUTPUT); pinMode(39,OUTPUT);
pinMode(40,OUTPUT); pinMode(41,OUTPUT);
REG_PIOC_OWER = 0xFFFF;
}
int i=0;
void loop() { // C.4 - C.9
REG_PIOC_ODSR = i<<4;
if (++i>64) i=0;
}
I really like the VHS quality on this, 40 columns of text and all that! That space-age, Z80/6502 look 
Will try with 82 ohm resistors, but unloaded the voltages seems pretty spot on, according to the composite video wiki page.