My comment was about the lack of easy extensibility of the codec-independent per-segment data (SegmentInfoPerCommit - info about stacked data is per-segment and per-commit), so LUCENE-3837 will need to use for now the codec-independent index-global data (SegmentInfos).
Well SegmentInfos is just the list of SegmentInfoPerCommit, so I think in the case of LUCENE-3837 we would just place this data alongside the only other per-segment-per-commit data: deletes (e.g. we would add something like updatesGen and updatesCount or whatever).
Sure, we have to bump the file header and what not, but the file is so simple now in the sense its just a list of segment names, the codec to decode them, with their deletes, that it wouldn't be a big deal.
And I think per-segment-per-commit data is pretty rare: we only have deletes, and in the future updates, but I can't imagine lots of other stuff belonging in this category (versions the per-segment metadata in SI which has been pretty volatile in the past).