Tenho interesse no tópico e fiz uns testes. Não sei se ajuda em algo, mas deixo o script pra consulta.
### <code r>
head(Orange, 3)
mForm <- as.formula("circumference ~ A/(1+exp((B-age)/C))"); mForm; class(mForm)
mExpr <- mForm[[3]]; mExpr; class(mExpr)
# A/(1 + exp((B - age)/C))
fm1 <- nls(mForm, Orange, start=list(A=160, B=700, C=350))
mPar <- summary(fm1)$parameters[,1]
xLim <- with(Orange, range(pretty(age)))
varx <- all.vars(mExpr)[!all.vars(mExpr) %in% names(mPar)]
sapply(1:length(mPar), function(i) {assign(names(mPar)[i], mPar[i], envir=.GlobalEnv)})
eval(call("curve", mExpr, xLim[1], xLim[2], xname=varx))
# curve(A/(1 + exp((B - age)/C)), xLim[1], xLim[2], xname=varx)
plot(circumference~age, Orange)
eval(call("curve", mExpr, xLim[1], xLim[2], xname=varx, add=T, col=2))
### </code>