What are some quantitative approaches to estimating credit risk for investments that aren't publicly traded, such as private equity and direct real estate? I'm particularly interested in estimating incremental changes in credit quality and probability of bankruptcy, and its implications for issuer risk. While I am familiar with approaches to this for publicly traded assets, such as structural models and stress tests, I haven't come across any literature applying these quantitative techniques to private investments.
How is this done in actual practice? I would be interested in any papers or literature reviews you've come across that tackle this problem.