There are no extenders that have been hacked to work with Sage to this point - that I am aware of. I have been following the Alternatives to Sage thread over on the Sage forums on and off since the Google purchase. Many have asked and some have found almost identical hardware, but the software/firmware for the extenders was never released in any form, so I doubt it will be replicated.
Most people who have purchased and are using Sage continue to say that it works fine, and they will continue to use it until it dies. I wish I was in that place, and had made the purchase before the acquisition by Google. Unfortunately, there doesn't seem to be another comparable solution that has comparable (small-form factor) extenders other than Next-PVR (aka NPVR). Unfortunately, the only extenders that work with NPVR have been discontinued (older model NMT's - A100 and A110) and are only available via Refurbishment orders through Popcorn Hour. From what I have seen on this, is that they post when they have a number of them available via their forum. This is not the way that I would want to procure extenders - particularly for a new system. What happens when one of mine fails? I couldn't easily order a replacement, and would have to send mine back for repair... not really interested in that hassle.
I played with NPVR and MediaPortal just a bit, as both are freely available (open-source). I had some issues with the setup of both programs, when trying to setup a single HDHR tuner. That alone was enough for me to stop working with either of them, as the HDHR is a fairly basic tuner that I figured enough people use that setup of one, and documentation of the setup should be easily available. That said, I think 7MC is probably the most 'polished' solution. I want to try it out... I have read many posts that mention issues with live-TV or lack of support for it (I havent completely followed this problem) and need to find out more about it. I really dont watch much live TV unless it's sports, and even that, I feel that I could deal with a non-real-time feed if that was available (e.g. time-delay or lag due to processing). The biggest problem I have with 7MC is the fact that the only extender is the XBOX - it's not small and would make me do some fairly substantial changes to use in my MBR or guest room as both of those TVs are wall-mounted LCDs with no shelving or place to set a XBOX. I have seen wall-mount solutions, but they don't hide the xbox well; I like simple/modern decor and adding a shelf for a XBOX extender is not something I want to do. Not to mention that XBOX's aren't CHEAP (if buying new) - particularly when compared to the original selling price of the Sage extenders.
Having said all of that... I think it is fair to repeat what others on the Sage forum have said... if there was a comparable alternative it wouldn't have take 30+ pages of discussion over on their forum. The problem seems to be that each software suite has it's own niche, whether PVR or media library playback, and none of them seem to be able to provide all of the features Sage offered - probably why Google bought them. My biggest problem, which I think it worth repeating, is the lack of alternative that has a small-form factor media extender (I don't want to be relative with the term small - I want an extender that can fit behind a wall-mounted LCD and go unnoticed).