Applies, compiles, works for me.
Very very minor comments that I should have noticed before, sorry for this additional round trip.
In the help line, move -I just after -i, to put short options in alphabetical and decreasing importance order. On this line, also add the information about the default, something like:
-i, --ini... .... -I, --custom....=[...]+ (default "ctgvp") ...
When/if the pgbench tap test patch get through, the feature should be tested there as well. No action needed now.