TWL_FIRM must be patched to allow that as that is what handles DS/DSi mode stuff. (GBA VC stuff uses AGB_FIRM and will need it's own patching as well). Currently it is not. Theoretically, Gateway could patch it in a manner similar to how they patch NATIVE_FIRM to allow unsigned 3DS stuff. Provided that the 3DS doesn't force a hard-reboot when launching DS/DSi mode (as gateway won't have a way of hooking into it unless they find a new exploit for TWL). If that is the case an exploit in boot rom would be needed and that probably won't ever happen.
But you'll be stuck with their standard "soon™" response on when to expect that to happen.