Do you have an exploited DSiWare app installed? If so you can downgrade your DS cart White list. You first have to get fwtool 1.6 and dump your nand, get the keys needed to decrypt it (FWTool can dump nand CID, consoleID can be retrieved by exporting a game to SD from Data Management and using dsi_srl_extractor on it. (that can be found in the DSi Downgrade thread)
You would then decrypt the nand, replce the SRL in the TWLN partition (via some file manager like WinImage or by mounting it via osfmount or some other virtual device mounter. Linux has native tools for this too) re-encrypt and then flash it back using fwtool.
Also if you go through the trouble of downgrading your system from 1.4.2 to 1.4, you can then use modified DS Cart White list which would allow you to white list flashcarts that use custom icons and what not. This may not allow booting really old carts that use bad arm7 ram/code entry addresses, but some carts like AceKard 2i on older internal fw to work.