Can (or should) the activities relating to collection of requirements for the Requirements Matrix be included in the Work Breakdown Structure (WBS)?
Yes. The WBS should include all the work needed to successfully complete the project.
The 100% rule states that the WBS includes 100% of the work defined by the project scope and captures all deliverables – internal, external, interim – in terms of the work to be completed, including project management. The 100% rule is one of the most important principles guiding the development, decomposition and evaluation of the WBS. The rule applies at all levels within the hierarchy: the sum of the work at the “child” level must equal 100% of the work represented by the “parent” and the WBS should not include any work that falls outside the actual scope of the project, that is, it cannot include more than 100% of the work… It is important to remember that the 100% rule also applies to the activity level. The work represented by the activities in each work package must add up to 100% of the work necessary to complete the work package. Effective Work Breakdown Structures By Gregory T. Haugan, Published by Management Concepts, 2001, ISBN 1567261353, p.17 (filched from wikipedia).
All work necessary for completion of the project, should be in the WBS. For completeness, I'll mention what I call the "decomposition threshold" - set a threshold of N hours; any activity that takes less than N hours should not be explicitly listed in the WBS, it should be part of other activities in the WBS.)
Alternatively, The Requirements Matrix is (should be) a deliverable for the project; it should be in the WBS.