Unfortunately, create_iam.c is totally useless in its current form. libiam.c is an empty shell calling into ldiskfs via ioctl() on the file to modify it via IAM code in the kernel, but the IAM_IOC_* ioctls no longer exist.
I had made a patch to use libiam to rebuild the FLDB file based on create_iam.c, but then discovered afterward tha libiam.c was useless. I ended up fixing the kernel parameter interface to be able to rebuild FLDB by using set_param. (Patch not pushed to master yet, as I was away on vacation).
I'm not against filling out libiam.c to actually be able to create IAM format files in userspace, but I suspect it would be a lot of work. On the plus side, latest libext2fs has added htree directory support (mostly for large lost+found), so that might be usable as a starting point.
Based on Andreas' message above it looks like the utility is useless now, so I am closing this issue are Won't Fix.