Is there a word for the particular usage of the curly brace, or a shape like it, to indicate part of a diagram?

Solution 1:

For the top one, you could say the curly brace is being used instead (or as) a dimensioning line.

Quoting http://www.design-technology.info/IndProd/page9.htm

Dimensioning The purpose of dimensioning is to provide a clear and complete description of an object. [...].
Definitions and Dimensions.
The dimension line is a thin line, broken in the middle to allow the placement of the dimension value, with arrowheads at each end (see diagram below). An arrowhead is approximately 3 mm long and 1mm wide. That is, the length is roughly three times the width. An extension line extends a line on the object to the dimension line.

The linked article has diagrams which make it clearer.