I posted about this in G+, but wanted to expand on the thoughts here.
It starts with this post from Greywulf.
The shared worlds thing was already done with Spelljammer. As I said in G+, Dragon Magazine even had an article for porting over Space:1889.
As far as writing style, I am all over the end to the Gamebooks are Technical Manuals school of thought. Even the rules to Life (boardgame) are not as dry as other game rules.
I don't like to use minis, so I'm just skipping that part.
I wanted to focus on the bit about doing this with an OGL game like Labyrinth Lord. To do one better in this exercise, why not just go straight to Microlite 20. One page of rules, many possibilities, right? Well, let's start with M20 Purest Essence. 17 pages.
The PHB: Remove all race and class information and the Game Master's Guide. Also remove the spell lists and the creature lists. What you're left with is about one page of rules to play the game.
The GMG: The entire M20 Purest Essence, although it will need to be organized.
Then we have the codices:
Humans - Race information for humans (+1 to all skills). Class information for all humans playing the various classes. Add the spell lists, but make them different in flavor and selection. The only place with a universal spell list is the GMG. That's just the crunchy stuff. The rest is history, a section like the Roman Way called the Human way. Outposts of humans in various game worlds, etc. Art, fluff, lots of stuff.
Elves - Race information for Elves (+2 MIND). Class information for all elves playing the various classes. Add the spell lists, but make them different in flavor and selection. You see where I'm going.
At the table, the player has a codex and a one page handout for rules. The GM has the GM Guide.
What do you think?