I think, based on viewing other topics here, that this is an unimplemented feature:
It is foundational in pattern design to have two edges which will be seamed together be of equal length. Currently, I don’t think there’s a way to create this relationship between, for example, two splines. It would be important for such a feature to accommodate updates to the measurement tables in a reasonable way.
I’m not a programmer, though I have some experience in programming. My thought is this: would it be possible to create a constraint on a new spline (maybe a new curve tool type?) which would force its length to be equal to another curve/line in the current project? I’d suggest that the software accomplish this by allowing the user to specify the start and end handle angles (tangents), and let the algorithm vary the “tension” of one or both handles (by choice) until the curve length requirement is met.
I realize that there are geometries for which this would fail outright, but experienced and careful pattern designers would get it to work, and get a lot of use from such a feature.