Six Sigma and Lean practices are applicable to any process, which has a life-cycle associated with it.

Software requirements analysis process has a life cycle as well. Software (S/W) Requirements start with customer needs, customer budget, application domain, compute environments, software functions, software architecture (preferred) , programming language (preferred) , software components, systems engineering, Software protocols (preferred) , Decompositions, S/W Modules, S/W- H/W Interfaces, S/W Modules Integration, Measurements, Unit Testing and customer validations, etc.

Once you establish a life-cycle process, you need to develop checklists, decompose into life-cycle elements,  aka modules, measure its behaviors, monitor its health, and manage the improvements by comparing the projected requirements or outcomes to the desired (target) or prescribed customer requirements.

Conference Paper Knowledge-based Enterprising (KBE) Strategy for Lean Product...

Basically, your question is about measuring a process. The answer is similar for any process but the performance values change. In other words, don't make it complicated.

For requirements, there is a general rule about eliminating vague words and sentences with multiple meanings. Indirect references and overloaded nouns are also bad. Since we want to remove these things we should have an estimate of how frequently they appear in source documents and how effective our removal processes are. There would also be a size/effort ratio for the effective process and a measure of cycle time.

You can find plenty of references on ambiguous lexicon and vague syntax.

The main objective for six sigma technique is reduce the variation of the process. which is 3.4 DPP. In the reality, it is not easy to get this index number, but it could to be close as much as to you can, depend on the the organisation target it and emplyee experiance and how much they help to get the target. On-line approcah used to apply six sigma which is DMAIC. 

 Remember Six Sigma it's like a toolbox, in the Define (Phase) tools I´m using the Kano Model, CTQ Drill Down tree and CTQ Workshops to define (first of all) the requirements (In software phase since the discovery phase you get operative / Process requirements), once I have the definition (Business blueprints) I´m using tools to design ( DFSS) and assure the compliance of all requirements. I think, the CTQ workshop; when you translate customer CTQ´s into Process KPI´s, its the most relevant tool I´m using in this phase.

You should use the DFSS methodology and tools to software development , if you are in AMS you can use the DMAIC to reduce the defects or to define enhancements.

Once you "adapt" the Six Sigma tools it could became a part of your methodology, Im working in methodology for development, security and ERP implementation. Pleas let me know how can we work together.