A couple of quick comments from a clinical perspective here (disclaimer: internist, not a nephrologist)
1) RE: measurement of true GFR. Rarely done. I've only seen it done in pre- kidney transplant evaluations. Uptodate mentions it might be done in some areas of chemotherapy due to narrow therapeutic windows of a particular drug, I've not seen it used in that context.
2) RE: eGFR. I believe an assumption in all equations is that the creatinine is in steady state. If you're going to use eGFR measures in a study, you'd want to parameterize any particular measure to ensure you aren't evaluating this in the context of a kidney injury and dynamically changing renal function.
Checking on a Cr measurement before and after (a suitable delta time) to ensure you're in a 'steady state' would be way to implement in practice?
This assumes the research use case does really need eGFR.
I do see a reasonable count in Stanford Stride v6 but none in v7 - I can ask around as to why this is if there's something moving forward....