I am also interested in integrating OMV with freeipa. I followed maximd8 instructions, although using OMV 6 in a virtual machine.
Just like donsimpson, joining the domain via ipa-client-install is successful and I get a list of local and ipa users using "getent passwd".
It is just that in OMV UI, the ipa users (and groups) are not shown in the user management section. Interestingly enough, the ipa users are shown in t he ACL dialogue of the shares!