Since the original article (2014 February), the future has arrived. DHCP 4.3.1, 4.2.7, and 4.1-ESV-R10 added DHCLIENT_DEFAULT_PREFIX_LEN in includes/site.h, and a user who wants a different default can edit this file and recompile. Of course the above script suggestion still works if you do not want to recompile (such as if using a distributor's package of ISC DHCP.)
We might offer a solution for runtime configuration in a future major release, but that has not yet been determined. The default for DHCLIENT_DEFAULT_PREFIX_LEN is still 64, but we might also consider changing that in the future.