Best way to rewrite uri to pass path segment as part of query string?

I haven't tested this yet, but you could try somthing like this.

when HTTP_REQUEST {
    if {[string tolower [URI::path [HTTP::path] 2 2]] matches_regex "[a-z]{1,}"}{
        HTTP::path "/[URI::path [HTTP::path] 3]"
        if {not ([HTTP::query] equals "")}{
            HTTP::query "[HTTP::query]&id=[URI::path [HTTP::path] 2 2]"
        }
        else {
            HTTP::query "id=[URI::path [HTTP::path] 2 2]"
        }
    }
}

e.g.

 irule

[root@ve11c:Active:In Sync] config  tmsh list ltm rule qux
ltm rule qux {
    when HTTP_REQUEST {
  set uri [HTTP::uri]
  if { [scan $uri {/%*[^/]/%[^/]%s} id rest] == 2 } {
    set rest [string trimright $rest "?"]
    if { $rest contains "?" } {
      set newuri "${rest}&id=${id}"
    } else {
      set newuri "${rest}?id=${id}"
    }
    HTTP::uri $newuri
  }
}
when HTTP_REQUEST priority 1000 {
  log local0. "$uri > [HTTP::uri]"
}
}

 test

[root@ve11c:Active:In Sync] config  tail -f /var/log/ltm
Apr 24 22:50:02 ve11c info tmm1[5649]: Rule /Common/qux : /prefix/foo/ > /?id=foo
Apr 24 22:50:10 ve11c info tmm[5649]: Rule /Common/qux : /prefix/foo/path > /path?id=foo
Apr 24 22:50:16 ve11c info tmm1[5649]: Rule /Common/qux : /prefix/foo/path? > /path?id=foo
Apr 24 22:50:22 ve11c info tmm[5649]: Rule /Common/qux : /prefix/foo/path?p=7 > /path?p=7&id=foo