I've just got this working, & I thought I'd share
So for anyone who has a jailbroken device , & who wants a sweet GBA emulator to take with them here's how to get it.
You'll need the big boss repo in your sources, It's a paid app, there are cracked versions on other repos I know but if you don't support the developer you won't get any more cool emulators!
Search for GBA.emu in Cydia. There are other GBA emulators available but I personally think this is the best. It also runs natively on iPad & has much better support for wiimote/icade/icontroller.
Download it!
use ifile to place your Roms into var/mobile/media/ROMs/GBA. This folder will have been created after installing the GBA.emu.
Open GBA.emu & select Load Game (top option) , navigate to the folder where your Roms are & select your game. That's it.
There are many other options in this such as button mapping & the orientation is automatically detected which I like. Full screen on the iPad too & the larger display actually works quite well. Playing portrait using th on screen controls works really well.
That's it. Simple. Note that your Roms must be .bin or .zip files to run.