RMarkdown/Bookdown: Putting R markdown/bookdown cross references into a plot legend in a R chunk

I have a bookdown document with many equations with equation numbers. I want to put an equation number reference in a plot legend - pseudocode

{Absorbance = D + \frac{A-D}{1+\left(\frac{Concentration}{C}\right)^{1/B}}}

(ref:eqLog) @ref(eq:Logistic4P)

legend('topright',legend=some reference to equation number)

Of course, the job is to get the bookdown equation reference into the r chunk, similar to putting a cross reference into a figure caption. Can someone point me in the right direction and just tell me it can't be done?