Oh, does it? That's quite interesting, I could understand why other 3DS projects don't use it then! Other versions of mGBA, like the Wii port also have that filter too.
I've tried OAF and it's going to be quite neat for those games that have issues trying to be played on other ways, I got a couple with severe glitches on GBArunner2, and reading about how it works was fascinating. Setting up the config file seems to be doable with GodMode9 and the Hex Editor, reading older pages of this thread on the 3DS itself, that's going to be useful for not having to go to a computer to edit it, although I only needed to change the backlight.
I've messed around with gpsp on Retro Arch, and I actually got a set up that could be considered on par with what mGBA does, or even slightly better. With these settings:
An Aspect Ratio [Custom 360x240] +
Bilinear turned: On +
Video Filter: Either Normal2x or Scalex2, depending on your tastes, I kinda recommend the latter.
You get as close as I could to what mGBA offers with Normalx2, but with Scalex2 the picture is a bit rounder but also sharper. As mentioned, not ideal by any means, and I've always said that people should try and play as they like so there's a lot of personal taste into this stuff...
But I'd argue that Scalex2 is the best I could make GBA look, it's the best compromise I could get between big picture and sharp/nice looking enough. Games with tiny sprites or text will suffer the most from non-integer scaling, but... Well, what can we do.
So... Games like, Final Fantasy I & II: Dawn of Souls with its tiny as heck fonts, doesn't look too hot. Final Fantasy Tactics Advance looks quite a bit better, and games with little text will benefit the most, but it's more of a game-by-game thing.
If anyone cares to try it, let me know what you think. Remember to go into Quick Menu, Overrides and Save Core Override so you don't have to go back and for tweaking settings repitedly.