At best, this might approximate "Open space, good condition, grass cover >75%" with a CN of 80. (From the TR-55 CN tables, HSG D)
Worst case is likely to be "...poor condition, grass cover <50%" with a CN of 89. (Same CN as HSG-D dirt roads.)
The exact type, depth, and condition of the mulch will defintitely influence where you fall in this range. But you may need to base the design on the worst-case (maximum) CN.