It would be interesting for the author to get a flashcart, load his rom on it, and add some video of it playing on physical GBA hardware. Then you can really find out if any quirks of the system would prevent it from running, or add any slowdown.
Unfortunately, parallel ports are exceedingly uncommon these days. However, it looks like this can be done easyly enough using something like a Teensy [0] or Arduino [1].
[0] https://github.com/tangrs/usb-gba-multiboot
[1] http://web.archive.org/web/20100815071014/http://blog.evildr...