> (%i7) :lisp (defun $my_formatted_output(a) (format t "~%~10,8f~% I am a new line~%" a))...
> Does not work with bigfloats.
> How to handle that case?
To go beyond the control you get with fpprec, you'd have to work a
little harder, since bigfloat is a Maxima-specific datatype. You
could try to rework the output of fpformat, or make a modified version
of fpformat itself. Then you can feed the string result to Lisp's
format.