The easiest way to use fontawesome icons with highcharter is using the the package with the same name by Richard Iannone https://github.com/rstudio/fontawesome because you can turn the icons into svg text or png files so you can use them in html tags or transform to data uri.

# install.packages("rsvg")
require(rsvg) # for fontawesome::fa_png
library(fontawesome)
library(highcharter)
library(stringr)

df <- data.frame(
  a = round(rnorm(10), 2),
  b = round(rnorm(10), 2)
)

fa_to_png_to_datauri <- function(name, ...) {
  
  tmpfl <- tempfile(fileext = ".png")
  
  fontawesome::fa_png(name, file = tmpfl, ...)
  
  knitr::image_uri(tmpfl)
  
}

# specify colors tu resue in the series/tooltips
rcol <- "#4C83B6"
pcol <- "#3CAB48"

rproj <- fa_to_png_to_datauri(name = "r-project", width = 22, fill = rcol)
pthon <- fa_to_png_to_datauri(name = "python",    width = 22, fill = pcol)

highchart() %>% 
  hc_title(
    text = str_c("This is a svg ", fa("rocket", fill = "#CACACA"), " icon"),
    useHTML = TRUE
  ) %>% 
  hc_add_series(
    df,
    "scatter",
    hcaes(a, b),
    name = "R icons",
    color = rcol,
    marker = list(symbol = str_glue("url({data_uri})", data_uri = rproj)),
    icon = rproj
  ) %>% 
  hc_add_series(
    df,
    "scatter",
    hcaes(b, a),
    name = "Python icons",
    color = pcol,
    marker = list(symbol = str_glue("url({data_uri})", data_uri = pthon)),
    icon = pthon
  ) %>% 
  hc_tooltip(
    pointFormat = str_c(
      "<b>",
      "<img style='vertical-align:middle' height='15' src='{series.options.icon}'/> ",
      "[{point.x}, {point.y}]",
      "</b>"
    ),
    useHTML = TRUE
  )

Another example inspired/copied from https://github.com/hrbrmstr/waffle:

library(dplyr) # to wokr with list columns
library(purrr) # to wokr with list columns

df2 <- tibble(
  type = c("fruit", "samminch", "pizza"),
  amount = c(46, 54, 80),
  faico = c("apple-alt", "bread-slice", "pizza-slice"),
  col = c("#d35400", "#907E4D", "#F89101")
)

df2
## # A tibble: 3 x 4
##   type     amount faico       col    
##   <chr>     <dbl> <chr>       <chr>  
## 1 fruit        46 apple-alt   #d35400
## 2 samminch     54 bread-slice #907E4D
## 3 pizza        80 pizza-slice #F89101
df2 <- df2 %>% 
  mutate(
    uri = map2_chr(faico, col, ~fa_to_png_to_datauri(.x, fill = .y)),
    marker = map(uri, ~ list(symbol = str_glue("url({data_uri})", data_uri = .x)))
  )

hchart(
  df2,
  "item",
  hcaes(name = type, y = amount),
  name = "What I eat",
  showInLegend = TRUE
  ) %>% 
  hc_plotOptions(
    # avoid hide series due bug
    series = list(point = list(events = list(legendItemClick = JS("function(e) {e.preventDefault() }"))))
  ) %>% 
  hc_legend(
    labelFormat =  '{name} <span style="opacity: 0.4">{y}</span>'
  ) %>% 
  hc_colors(pull(df2, col))

More details in item chart in this link https://www.highcharts.com/docs/chart-and-series-types/item-chart.