So since the Teensy++ 2.0 bootloader is not available and I'm not feeling comfortable to learn all the bells and whistles of USB communications I was looking for other options. Googling "at90usb1287 arduino" I found this https://github.com/mattairtech/ArduinoCore-avr - I chose the AT90USB1287 deliberately because of the native USB and fast speeds and way too many IOs to work with, and got it to work with my board :)

https://www.mattairtech.com/index.php/development-boards/mt-db-u6.html

I basically had to add the link to the json file to my boards manager library in th Arduino IDE and download the bootloader from their page (burn bootloader from arduino didn't set the fuses right?). I started with turning lights on and off, because as always blink is a good point to start. I noticed that my register setting skills were non existing, because I always mix up port direction and pin setting, but in the end I found the right way to set the ports and blink the lights. Reading the data port was also easy, after testing and finding out that the JTAG on pins F4-F7 weren't deactivated like the mattairtech folks promised.

Anyway... boom. Single ROM bank cartridges, fear me! Here's the verbose header dump of one of my TETRIS cartridges. Bank switching and writing cartridges to be next. It's fast. Off to bed.

100: 0 - 
101: C3 - �
102: 50 - P
103: 1 - 

104: CE - �
105: ED - �
106: 66 - f
107: 66 - f
108: CC - �
109: D - 

10A: 0 - 
10B: B - 
10C: 3 - 
10D: 73 - s
10E: 0 - 
10F: 83 - �
110: 0 - 
111: C - 
112: 0 - 
113: D - 

114: 0 - 
115: 8 - 
116: 11 - 
117: 1F - 
118: 88 - �
119: 89 - �
11A: 0 - 
11B: E - 
11C: DC - �
11D: CC - �
11E: 6E - n
11F: E6 - �
120: DD - �
121: DD - �
122: D9 - �
123: 99 - �
124: BB - �
125: BB - �
126: 67 - g
127: 63 - c
128: 6E - n
129: E - 
12A: EC - �
12B: CC - �
12C: DD - �
12D: DC - �
12E: 99 - �
12F: 9F - �
130: BB - �
131: B9 - �
132: 33 - 3
133: 3E - >
134: 54 - T
135: 45 - E
136: 54 - T
137: 52 - R
138: 49 - I
139: 53 - S
13A: 0 - 
13B: 0 - 
13C: 0 - 
13D: 0 - 
13E: 0 - 
13F: 0 - 
140: 0 - 
141: 0 - 
142: 0 - 
143: 0 - 
144: 0 - 
145: 0 - 
146: 0 - 
147: 0 - 
148: 0 - 
149: 0 - 
14A: 0 - 
14B: 1 - 

14C: 1 - 

14D: A - 

14E: 16 - 
14F: BF - �