Browse the implementors of #beVariable message and you will find other filters under BPatternVariableNode class, such as #beInstVar or #beLocalVar. If you miss something, just add a method. No new syntax required.
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You might assume this pattern is inherent to streaming. It isn't. The reader acquisition, the lock management, and the { value, done } protocol are all just design choices, not requirements. They are artifacts of how and when the Web streams spec was written. Async iteration exists precisely to handle sequences that arrive over time, but async iteration did not yet exist when the streams specification was written. The complexity here is pure API overhead, not fundamental necessity.
The efficiency depends on the query size relative to the data distribution. A small query in a sparse region prunes almost everything. A query that covers the whole space prunes nothing (because every node overlaps), degenerating to a brute-force scan. The quadtree gives you the most benefit when your queries are spatially local, which is exactly the common case for map applications, game physics, and spatial databases.