Hi Lemonjuice,
I am no expert on philosophy, however, I would say that given the nature of pragmatism and mixed methods, i.e. coming from a practical viewpoint, with the focus on the end goal, it would be impossible to capture an ontological position. Although the number of texts using epistemology and ontology as one and the same does not help.
If you want to read a very accessible book on pragmatism,. I would recommend Talisse and Aikin (2008) Pragmatism: A guide for the perplexed. In Chapter 2 they deal with this issue at considerable length. That really helped me. As my PhD was also mixed methods, based on pragmatism.
Best of luck.