Would something like this work?
library(dplyr, warn.conflicts = FALSE)
library(dbplyr, warn.conflicts = FALSE)
#> Warning: package 'dbplyr' was built under R version 3.5.2
df <- memdb_frame(x = runif(100), y = runif(100))
df %>%
mutate(x = y - !!build_sql("interval '12 months'")) %>%
show_query()
#> <SQL>
#> SELECT `y` - interval '12 months' AS `x`, `y`
#> FROM `tyziaqugiu`
Created on 2019-01-22 by the reprex package (v0.2.1)