-tck_weights_out options can indeed be “hijacked” for this purpose.
The streamline weights in particular require their own explicit command-line options, since those weights are interpreted in a very specific way in certain areas of the code. I have however considered in the past how it might be possible to handle other use cases: Described here was keeping track of the streamline indices, but alternatively (or maybe the ultimate purpose of such?) there may be some arbitrary quantitative parameter per streamline that one wishes to retain correspondence with the associated streamlines, even though some subset of streamlines are being extracted.
There’s a number of different ways that this could be achieved in the software, each with their pros and cons. Bumping this thread in case anybody out there has current or potential use cases where such capabilities would be useful that may prove informative, or opinions on how such data could be most intuitively managed from a software interface perspective.