Perhaps you could set up control logic that determines with measure is used?

bmiCovariate <- "bmi"

if(bmiCovariate == "bmi"){
  
  tv <- tmerge(data1=tv, data2=visitWise, id=personId, 
               bmiMeasure=tdc(visitDay, bmi))
  
} else {   
  
  tv <- tmerge(data1=tv, data2=visitWise, id=personId, 
               bmiMeasure=tdc(visitDay, modifiedBmi))
  
}