Stever's Solution

Nub code /select/ {
    <form> action /deliver/ method post [subst {
	[<fieldset> fs title "Select Item" {
	    [<text> number legend "Item Number:" {}]
	    [<submit> submit Ok]
	}]
    }]
}

Nub code /deliver/ {
    dict set r -Query [Query parse $r]
    set query [Query flatten [dict get $r -Query]]
    set number [dict get $query number]

    # ... TODO run the process as an [open |prog], consume the input and call [done $r $content]

    return [Httpd Suspend $r 100000]
}

proc done {r content} {
    dict set r -content $content
    dict set r content-type application/pdf
    Httpd Resume $r
}