It could be as simple as mirroring wikipedia but never propagating the deletions. Trying to edit an article would redirect one to wikipedia; trying to edit a deleted article would instead edit the local copy (in the event of the article being recreated on wikipedia at a later date there could be an automatic upstream merge).
If you took half a second to look through the "New pages" feed, and saw not only the hideous quality and dubious nature of nearly all newly created pages, but also the vast quantity of new articles created every few minutes, you might rethink your position on keeping all deleted articles. Many new articles deserve speedy deletion.
Ok, so only add pages to the mirror that are older than N days or have more than M (non-reverted) edits. Probably you'd also need to have tracking for when page names change. It's not a deal-breaker.
