Service to underline plain text with a line of dashes
BBedit can execute shell commands on selected text, so adapting something like
echo 'This is a line of text' | sed -n 'p;s/./-/gp'

to make it work when called from BBedit should work.

sed -n runs sed without echoing its input automatically
p is the first command applied to the input, it just prints the input (technically the content of the pattern space)
s/./-/gp is the second command, it replaces all characters in the pattern space by - and prints the result