2) There is no formatter to force exponential notation output corresponding to ~E in Common Lisp, %e/%E in C, and Fortran D, E, and F.  I suggest numeric/float.  The following 4 additional parameters would be needed:

a) integer-width: specifies the number of integer digits (default 1; 0 means use a single digit that is always 0.

b) exponent-width: specifies the number of exponent digits.

c) exponent-flag: specifies the character separating the exponent from the mantissa (default E)

d) exponent-multiple: specifies a number such that the exponent is always a multiple of this number, so that 500000 comes out not 5e5 but 500e3.

3) numeric/si: allow only 1024 or 1000 as bases, as the other bases are just unintelligible.  Better yet, change to a boolean variable.

4) fl: what happens when the line position is not known, as when the last output was via an R7RS-small output procedure?

5) nothing: this name conflicts with my forthcoming Maybe/Either SRFI: I suggest changing it to "silence" even though this is a breaking change with SRFI 159.

6) editorial: Need a blank line above joined/last.

7) Pretty printing: specify Emacs compatibility, or else deviations from it.