The 3DS is missing a few key files on TWLN Partition that is needed for DSi System Menu. At one point I got those files installed and almost got DSi System Menu to boot. I might be able to launch it via Unlaunch, however Unlaunch doesn't recognize or is unable to init the Wifi on 3DS and doesn't let me proceed any further. That aside System Menu either white screens or shows an error screen when I try to force it.
The issue is the dev launcher/boot2 combination that TWL_FIRM uses results in the stage2 SRL I use thinking the console is a HNAA region console. Thus I can't use a HNAE (USA region) retail DSi Launcher SRL without modifying it's header and System Menu doesn't like to run with a modified header so no go on that.
So yeah don't get your hopes up on that.
As for getting the DSi bios files. (For No$GBA?). You are better off dumping those off a real DSi. I think the ones the 3DS has are incomplete. Especially the DS bios files. You won't be able to boot DS firmware menu with the dumps from 3DS (or a DSi for that matter). For DS firmware files it's best to get those off a real DS/DS Lite.
I would really like to run the DSi Menu on my 3DS... I have all the required files, could it be done by swapping the DS Download Play in TWLNAND with the DSi Home Menu and also copying everything else?