In general jq maps very well to any hierarchical datastructure. One of the maintainers has made 'fq' which supports BSON, MsgPack, Protobuf, CBOR and even media files png, jpg, mp4 etc.

SQLite when compiled for size is 590 kB. But I think a full jq database implementation based on Lite³ would be possible under 100 kB.

Lock-free shared state relies on algorithms that can make clever use of atomic instructions. But you should not have threads write to the same memory regions, because the hardware only allows for 1 core to have a cacheline in a writeable state. If another core attempts a write to the same line, this will immediately invalidate all the other copies. Under high contention the coherency penalty becomes so large that throughput falls through the floor. So basically the algorithms need to do most of the work in separate memory regions, then occasionally coordinate by 'committing' their work via a spinlock or similar.

Lite³ implements some settings for node alignment, but not for user data. It would be possible to create a bytes type with extra alignment guarantees.