i had figured that tes 3 and 4 might not be considerable due to using outsourced software development kits. on the other hand, quake 3 uses punkbuster, and id simply stripped the punkbuster code before releasing. i understand that none of the numbered elder scrolls titles may be able to compile due to external dependencies that are not legally obtainable for personal use, but that's half the reason to release them, allowing independent developers to replace these closed source proprietary code compilations with free libraries
It's one thing to remove the speedtree, havok, or facegen add-ons to Oblivion, but GameBryo makes up the foundation of the Morrowind and Oblivion engines. The GameBryo code was also heavily modified and augmented and that would make it extremely difficult to remove the GameBryo bits and leave the Bethesda code.
Bethesda doesn't seem adverse to releasing source code, they recently allowed Arkane to release the Arx Fatalis code and id also has a history of these releases. The fact that Bethesda itself hasn't released anything like this suggests to me it isn't feasible.