How can I color the border line differently between two points on a map made with geom_sf and ggplot2?

Recently, I have posted a similar question here and I received the following answer,

library(sf)
library(ggplot2)

nc <- st_read(system.file("shape/nc.shp", package="sf"))

rect <- list(matrix(
    c(-85, 35.5, 
      -80, 35.5, 
      -80, 36.5, 
      -85, 36.5, 
      -85, 35.5),
    byrow = TRUE, 
    ncol = 2
  )) |>
  st_polygon() |>
  st_sfc(crs = st_crs(nc))

border_line <- st_intersection(
  rect, 
  st_cast(st_union(nc), "MULTILINESTRING")
)

ggplot(nc) +
  geom_sf() +
  geom_sf(data = border_line, color = "red")

which produced this map:

and it worked well at that time. However, I have recently been searching for a more elegant solution to this problem, as the aforementioned answer can be somewhat troublesome, especially when dealing with border lines that have conflicting latitudes and longitudes, which the northeast region of the attached map can be an example for this case. Therefore, I do welcome your suggestions and thank you in advance for your attention.

Something like this may work for you. First define approximate start and end co-ordinates of the section you want to colour:

start <- c(-85, 35.5) |>
  st_point() |>
  st_sfc(crs = st_crs(nc))

stop <- c(-81.8, 36.6) |>
  st_point() |>
  st_sfc(crs = st_crs(nc))

Now get the whole border as a collection of points, and find which points are nearest to your chosen start and end points:

border_points <- st_cast(st_union(nc), "POINT")

start <- which.min(st_distance(start, border_points))
stop <- which.min(st_distance(stop, border_points))

Now subset the border points between the start and end markers, and convert them into a single linestring:

border_line <- border_points[start:stop] |>
  st_coordinates() |>
  st_linestring() |>
  st_sfc(crs = st_crs(nc))

You can then plot as before:

ggplot(nc) +
  geom_sf() +
  geom_sf(data = border_line, color = "red")

This should handle irregular coastlines a bit better than the bbox method. For example, the East of the nc map could be marked as follows:

stop <- c(-76, 36.4) |>
  st_point() |>
  st_sfc(crs = st_crs(nc))

start <- c(-78, 34) |>
  st_point() |>
  st_sfc(crs = st_crs(nc))

border_points <- st_cast(st_union(nc), "POINT")

start <- which.min(st_distance(start, border_points))
stop <- which.min(st_distance(stop, border_points))

border_line <- border_points[start:stop] |>
  st_coordinates() |>
  st_linestring() |>
  st_sfc(crs = st_crs(nc))

ggplot(nc) +
  geom_sf() +
  geom_sf(data = border_line, color = "red")