Yes there is some equipment to do transcutaneous pCO2 but that's mostly used in sleep studies AFAIK. Most of the time you use a blood test. I heard in some research they use IR on expired air not sure how common that is. If you find a "home made" way to get a (even just relative) pCO2 reading I would be highly interested.

The desire to breathe in a healthy individual is a really good proxy for CO2 levels based on a wonderful design with distributed sensors throughout the body.

It is not a good proxy for O2 levels in healthy individuals, but may be in people with COPD or other chronic breathing problems.