Standard evaluation in dplyr: "unable to find function" error for functions in a global environment

Question

I am trying to use the standard evaluation in dplyr with functions in a global environment, but I am getting a "cannot find function" error. Here is the code

# create data.frame
df <- data.frame(x = rnorm(10), y=rnorm(10))
# define arbitrary function
test <- function(x) x^2
# use standard evaluation with dplyr
df %>% mutate_("mean(x)")
df %>% mutate_("test(x)")
# Error in mutate_impl(.data, dots) : could not find function "test"

I can get around this problem using df %>% mutate_(~test(x)), but I would like to use a string approach for various reasons. The main reason is that the user can pass a vector of variable names xto my function. I can use this vector to apply the same transformation to all variables, passing something like this as.list(sprintf('mean(%s, na.rm=TRUE)', x))to .dots. Here is the complete code

x <- c('x','y')
df %>% mutate_(.dots=as.list(sprintf('mean(%s, na.rm=TRUE)', x)))
df %>% mutate_(.dots=as.list(sprintf('test(%s, na.rm=TRUE)', x)))

, - ? , ? , ?

sessionInfo() output

R version 3.1.1 (2014-07-10)
Platform: x86_64-apple-darwin13.4.0 (64-bit)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] magrittr_1.5            dplyr_0.3.0.2           plyr_1.8.1              maptools_0.8-30         sp_1.0-16               Rcpp_0.11.3            
 [7] matrixStats_0.10.0      rgeos_0.3-8             stringr_0.6.2           mvtnorm_1.0-0           RcppArmadillo_0.4.400.0 Defaults_1.1-1         

loaded via a namespace (and not attached):
[1] assertthat_0.1    DBI_0.3.1         foreign_0.8-61    grid_3.1.1        lattice_0.20-29   lazyeval_0.1.9    parallel_3.1.1    R.methodsS3_1.6.1
[9] tools_3.1.1