- In continuation-mark-set->list*, perhaps the fixed value of #f to
  indicate a missing value for a mark could be replaced with a more
  general default, similar to what is used for hash tables in SRFI 69,
  etc. (so that programs may easily use #f as a mark's value).  A
  similar comment applies to continuation-mark-set-first and
  call-with-immediate-continuation-mark.

Racket provides such default values. I stripped it down for simplicity so that SRFI 157 mainly includes the core concepts. You can still use #f as a mark's value as follows:

(with-continuation-mark <key> #f (with-continuation-mark 'dummy #t ...))

In ... you can then in (continuation-mark-set->list* <marks> '(<key> dummy)) whether #t shows up in the dummy slot.

Have you taken a look at the more general Racket procedures? If you think it is worthwhile I am adding default values to SRFI 157.