callit <- function(.fn,...){
do.call(what=.fn,args=list(...))
}
library(dplyr)
(function(a, b){paste0(a, b, collapse="")}) %>% callit(a="x", b="y")
however this is just :
(function(a, b){paste0(a, b, collapse="")})(a="x", b="y")
with more steps (though it would allow you to inject additional functionality from callit (but you didnt mention needing to do that).
Additionally I would assume that if this is about plumbr functions, they would be named functions, so the focus on anonymous functions seems very odd to me, but good luck to you !