Thanks for the suggestion Perry.
Unfortunately it wouldn't work for several reasons.
First view-engine has no way to communicate with memcached currently.
Second, it would slow things down significantly.
Third, how could that work for reduces? For precomputed reduce values, which is the strength of couchdb's btrees + mapreduce, how do you "unreduce", exclude values produced for expired documents, and re-compute reductions? Not only you would need to know the map values produced by expired documents, you would also need to know the map values for the non-expired documents. Not to mention the big performance penalty here.
There's a lot of other technical issues that would impact either correctness, the incremental view update approach or performance. Those 3 listed above are just the ones people in general not familiar with implementation/design would grasp quickly.