For just capturing the output of a command, you can use:

r !somecommand

But if you want to filter a range of lines through an external filter with POSIX ed(1), you need some sort of temporary storage (whether a file, the system clipboard, a tmux buffer or whatever). There's no analog to

:10,20! rot13

offered by vi/vim.

Someone (can't find the post/author at the moment) did hack this functionality into ed, but it's a custom fork of POSIX functionality.