That is very unusual.
A fair bit is know about the development kits for the GBA and that does not look like one of them, it could be for a rarer type (most of what we see if from companies with dev kids, Nintendo and all the others often having their own secret sauce versions).
http://devkits.handheldmuseum.com/GBA_Wideboy.htm
http://i.imgur.com/SGvSuvd.jpg
Likewise all the homebrew peeps used multiboot cables to get data back from the GBA. I don't know what was officially used (though some official devs used multiboot cables and unofficial emulators as apparently they were better), That particular colour/style of plastic does scream prototype to me, though I have seen it on end user things in the past. That is all moulded around it and looks like it once had a cover would be more official than most mods I see done, though at the same time it is easily doable if you are going to make your own plastic cases. Speaking of the moulding it is a bit different to the commercial GBA I have here around the battery connector, I can not see what screws it uses but others playing along at home mine does have a cross screw in that location.
Compared to a standard motherboard the connector is quite far away from the CPU and memory, though at the speeds involved it might still have worked. 10 pins on one side, possibly then doubling to 20, is possibly in that middle ground between enough to do something with and enough to only do something basic.
So yeah I am stumped. While I am certainly not the most knowledgeable about the various GBAs out there I would like to think I don't do badly. Maybe someone else will know it.
Alternatively if you can take the back off so we can see where the connector leads (if it goes to controllers it is some kind of external controller, possibly prior to the GB player or that wideboy thing I linked, then there is that, if it goes to the CPU/memory then yeah, I doubt it will be a video out but why not.
Give me a little bit and I might stick this up on the portal. Been a while since we had such a thing.