Contrary to what some may tell you, it is possible to access TWL (technical name for DSi/DS carts/mode) carts using software while running in CTR mode (technical name for 3DS). (and you don't need any Arm9 magic to make it happen. Pokebank certainly runs fine without Arm9 exploits last I checked.
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Example being that Pokebank is able to access and remove saved pokemon on Pokemon Black/White and Pokemon Black2/White2 which are both TWL titles. You just have find out what service calls that app is using and adapt them for homebrew. Although full dump/write of DS carts might be a bit more tricky. Pokebank only really access certain areas of TWL carts and doesn't need to wipe/read the entire save of the games. Then again this may not be an issue.
I guess no one really cares enough about DS homebrew anymore to bother trying to get save backup/restore working for TWL titles. The closest there is, is twlnandfiler which is a dev title made by Nintendo for dev units for accessing DSiWare saves. But that title runs in TWL mode (specifically DSi mode) and is also a dev app which retail TWL_FIRM will refuse to boot. So unless you got a dev unit or get someone to dump TWL_FIRM from a dev unit (and hope it will work correctly on a retail unit), you won't be using that app.
I recall awhile back getting someone who had a dev unit to screenshot what twlnandfiler's GUI looks like and from what I can tell, it does not have access to slot-1, so trying to get twlnandfiler to work may be a waste of time if you aim to use it for slot-1. (besides, TWL_N partition can now be decrypted using TWLTool giving you direct access to the full TWL nand reserved for DSiWare so that you can backup/replace save files for DSiWare.
I don't know if apps running from CTR mode have access to TWL_N partition. Arm9 might be setup to prevent that but as far as I can tell Arm9 isn't blocking anything from accessing TWL carts from slot-1. Not sure about rom dumping though. You may need modified DSiWare in a hacked TWL_FIRM to do that. No one cares enough about DS mode to bother trying to find a way of patching out sig checks for TWL_FIRM.