You just pass it as value of the call. I suspect the reason it doesn't work is in your code, not in the facility (note that the link above is useless since the construction is mystery - if you were constructing it right, it would work ;)).
Small example:
SEXP myEval(SEXP FN, SEXP first_arg) {
return eval(LCONS(FN, CONS(first_arg, R_NilValue)), R_GlobalEnv);
}
... or reading R-ext:
"There are a series of small macros/functions to help construct pairlists and language objects (whose internal structures just differ by SEXPTYPE. Function CONS(u, v) is the basic building block: is constructs a pairlist from u followed by v (which is a pairlist or R_NilValue). LCONS is a variant that constructs a language object. Functions list1 to list4 construct a pairlist from one to four items, andlang1 to lang4 do the same for a language object (a function to call plus zero to three arguments). Function elt and lastElt find the ith element and the last element of a pairlist, and nthcdr returns a pointer to the nth position in the pairlist (whose CAR is the nth item)."
Reading R-exts,
-1- I believe it would be a good idea to put an example with CONS and LCONS in section 5.11.
-2- Building a LANGSXP list with tags from arguments needs invocations of CONS LCONS SET_TAG and install. While this is not exactly to my taste, using install is really not to my taste since it checks the symbol table and eventually creates a symbol. Isn't there a way to create a tag without using install over and over? A macro that simply sets the tag to whatever CHARSXP might be useful?