R: plm - limiting coefficients

Question

I want to limit some factors in the plm model in R. For example, if I have a model like this:

library(plm) 
data("Produc", package = "plm")
zz <- plm(log(gsp) ~ log(pcap) + log(pc) + log(emp) + unemp,
      data = Produc, index = c("state","year"))

How can I set the log (pcap) coefficient to -0.05?
There are type arguments restrict.matrixand in plm restrict.rhs, but they do not work.
I have all the ready-made codes that I widely use to model data. The plm object is used for forecasting and testing, so I would like a solution that works with the plm object directly and can be used later in my codes.

Thanks for any help!