How to change location and name of files generated by engines in Rmarkdown in each chunk

Question

I setup an engine on RMarkdown:

knitr::knit_engines$set(mplus = function(options) {
code <- paste(options$code, collapse = "\n")
fileConn<-file("mplus_code.inp")
writeLines(code, fileConn)
close(fileConn)
out  <- system2("/opt/mplus/8.10/mplus", "mplus_code.inp")
fileConnOutput <- file("mplus_code.out")
mplusOutput <- readLines(fileConnOutput)
knitr::engine_output(options, code, mplusOutput)
})

I am running it as follows:

```{cat bcfa, engine.opts = list(file = "assets/code/mplus/mplus_bcfa.inp", lang = "mplus"), name = "mplus_bcfa.inp"}
TITLE:  TWO-FACTOR MODEL OF POLITICAL ATTITUDES
DATA:
FILE IS assets/data/BAYESCFA.dat;
VARIABLE:
NAMES ARE X1-X10;
DEFINE:  STANDARDIZE X1-X10;
ANALYSIS: ESTIMATOR=BAYES;
      PROCESSORS = 10;  ! used to increase computational speed
      FBITER = 10000;  ! over-rides Mplus convergence criterion
MODEL:
 CONSERV BY X1* X2 X3 X4 X5;
   LIBERAL BY X6* X7 X8 X9 X10;
   CONSERV-LIBERAL@1;
! SPECIFICATION OF CROSS-LOADINGS:
   CONSERV BY X6-X10*0 (A1-A5);
   LIBERAL BY X1-X5*0 (B1-B5);
MODEL PRIORS:
    A1-B5 ~ N(0,.01);
OUTPUT: TECH1 TECH8 STAND;
PLOT:   TYPE=PLOT2; 
```

In other words, this chunk will write the syntax in the assets/code/mplus/mplus_bcfa.inp file and show it on the xaringan slides with the Mplus format highlights.

I would like to call that chunk and run it, generating the output file with the name and in the location decided by me. I tried it using this following chunk (for the next slide) so it shows the output:

```{mplus, eval=TRUE, echo=F, ref.label='bcfa', root.dir = "assets/code/mplus"}

I would like this chunk to create the file mplus_bcfa.out in the folder assets/code/mplus. Although, it does not do it, it just creates the file mplus_code.out (as define in the knitr engine configuration) and places it in the root of the RMarkdown folder.

To add some more context, I tried also to define input and output within the engine options:

knitr::knit_engines$set(mplus = function(options) {
 code <- paste(options$code, collapse = "\n")
 fileConn<-file(options$input)
 writeLines(code, fileConn)
 close(fileConn)
 out  <- system2("/opt/mplus/8.10/mplus", options$input)
 fileConnOutput <- file(options$output)
 mplusOutput <- readLines(fileConnOutput)
 knitr::engine_output(options, code, mplusOutput)
})

And then call then within the chunk options (i.e., input and output:

```{mplus, eval=TRUE, echo=F, ref.label='bcfa', root.dir = "assets/code/mplus", input = "assets/code/mplus/mplus_bcfa.inp", output = "assets/code/mplus/mplus_bcfa.out"}
```

But it did not work...