Can't think of one that would let you interactively edit it directly on the command line but there are ways to make it more convenient than what you're describing. If it's a short term change and you just need to add something to the path, you can self reference the current value. For example, in Bash:
Or if you want to alter precedence, you can prepend it with
You can also save yourself a cut and paste by echoing the value into a temporary file
echo $PATH > tempfile.txt
If you have more complex or permanent changes to make, you can reference and edit your environment variables in your equivalent rc and or profile file. If they're not already present, you can use the echo method to get them in an editable state, (just make sure to use >> to append instead of overwrite). Or, in vi, you can also use
In emacs it would be
C-u M-! echo $PATH
To insert the output into the file you're working on.