Where can I find references that directly relate drag coefficient increases to passenger capacity reductions?
The book "Airplane Design Part I: Preliminary Sizing of Airplanes" by Jan Roskam covers this extensively.

Nothing tabulated, mind you but the text goes into extensive detail of the equations involved for the sizing of a prospective aircraft and the relationships between the aircraft range, drag polars under various flight conditions and payload weights. 

It also gives the equations for calculating the sensitivity of the change of aircraft range with drag polar and payload weight, which is essentially what your passenger is.

EDIT

Relevant section is in chapter 2 from page 74 onwards.Problem 1 on page 85 is practically custom tailored for your needs here.

Bear in mind there is no one size fits all application of a rule of thumb for this question as every aircraft will differ on account of any number of factors.