I see that you've invested a lot of time in stackable Eviction Strategies.
That's good for me, because that means, that you've already made the first step : admitted that the default LRU is not as good as it could.

Perhaps you would like to incorporate this changeset:
https://groups.google.com/forum/?fromgroups#!topic/memcached/MdNPv0oxhO8
which makes sure that expired items are never in memory, and does so in O(1).
We use it in nk.pl since years at it works great (evictions dropped to 0, and monitoring memory consumption provides more information now -- also, slabs now have a chance to become emptied and disposed).

The only drawback I can see with it is the additional O(1) memory per item for doubly linked list pointers. I believe that Twitter hires some tough hackers which could make this number smaller (how about the trick with XORed pointers?).