In emacs24 -q,

    M-: (require 'jack)
    M-x jack-start

gets an error

    Wrong type argument: consp, nil

The debugger suggests it's

    (setcar (nthcdr 16 mode-line-format)

I wonder if one of the existing mode line variables could be used
instead.  Maybe mode-line-process, except perhaps extended to show if
the sub-process has died.

    (setq mode-line-process `(:eval (format " load:%.2f" jack-load)))