Here is a small patch that you can apply to the DSi Download Play app on DSi emuNAND that will disable RSA signature checks for downloaded apps. This will let you receive unsigned (e.g. modified) Wi-Fi multiboots in Download Play mode. For instance, you can use this to play modified New Super Mario Bros. levels in Single Card Multiplayer, or a translated version of Mega Man Star Colosseum (not available in the currently released version of the Rockman EXE OSS translation). I ported this from...
RSA patch for DSi Download Play
RSA patch for DSi Download Play