iRule for LTM to send HTTP Error code

The curly braces prevent variable / command interpretation. You could use subst (http://www.tcl.tk/man/tcl8.4/TclCmd/subst.htm) or escape any double quotes and not use braces to include a command in a value. I haven't tested this but something like it should work. Note that I've set the HTML in a variable once in RULE_INIT rather than generating the HTML on every request.

when RULE_INIT {
    HTML content to send.
    Need to escape double quotes, literal square braces and other characters?
    Example includes an escaped line break to shorten the long line.
   set static::custom_html "My Title with escaped \"double quotes\"\
   \[escaped literal square braces\]
500 spaces here: [string repeat 500 " "]}] 
}
when HTTP_REQUEST {

    Send the HTML
   HTTP::respond 200 content $static::custom_html
}

Aaron