I could be wrong but I didn't see headers, make files, etc. in the freeradius centos rpm.
Correct, because, as John Dennis from RedHat and others have pointed out, earlier versions of FreeRADIUS were not multilib-safe to provide that on CentOS 6 (which is based on RedHat Enterprise Linux, but you know that already). However, John is changing this with FreeRADIUS 3.0 (there's a pretty freeradius-devel package that will be available, which will include the necessary bits). So you're going to have to wait for FR 3.0 and its associated packages for RHEL/CentOS7 or Fedora 20. I'm following John's work on this closely because I will require FR 3.0 on CentOS 6.4 myself.
The one idea I had was to compile the module during our install process using the rpm installed freeradius.
Once freeradius-devel is available, you probably can get your install process to install it, do your build, and after successful compilation, uninstall freeradius-devel again. You'll just have to be patient. If you don't want to wait, you don't have a choice and the tarball it is. Stefan -- This e-mail and any attachments may contain confidential, copyright and or privileged material, and are for the use of the intended addressee only. If you are not the intended addressee or an authorised recipient of the addressee please notify us of receipt by returning the e-mail and do not use, copy, retain, distribute or disclose the information in or attached to the e-mail. Any opinions expressed within this e-mail are those of the individual and not necessarily of Diamond Light Source Ltd. Diamond Light Source Ltd. cannot guarantee that this e-mail or any attachments are free from viruses and we cannot accept liability for any damage which you may sustain as a result of software viruses which may be transmitted in or with the message. Diamond Light Source Limited (company no. 4375679). Registered in England and Wales with its registered office at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom