Escape is a script that displays lines in your scrollback in escaped form, so you can copy and paste them including colors and special characters.
Do /escape (or /esc) just like you would use /lastlog
this then prints the lines, which you can use in /eval.
see /help lastlog for options
Hey! join #fooX (where X is some char you can't copy-paste)
then you do /esc #foo
and you'll get something like 19:50 \x048/<\x04g \x04gsomeone\x04g\x048/>\x04g \x04ehey! join #foo\x12
ignore the stuff in the first part of the message, that's irssi's internal color format for the line printed.
doing /eval join #foo\x12 will make you join the right channel
/esc -clear makes your scrollback clean again