The second tutorial does say at one stage that if you perform the Dev BIOS mod you should be able to boot Dreamshell directly from the SD card although having never used it or modified my Dreamcast I'm not sure if you can then use Dreamshell to load your homebrew software from an SD card as well.
I do remember a piece of hardware being designed and manufactured to replace the disc drive and do exactly that though called GDEmu so that may be worth a look.
In an ideal world though I have to agree with Ferral and say that I'd probably just replace my drive and keep my Dreamcast fully functional for those rare but fun occaisions when there's a few people round that fancy some Soul Calibur or Powerstone multiplayer action