Essentially, Pi-hole becomes the local network DNS server where it parses all client lookup requests for "port 80". If the lookup is in an ad-domain and others, that particular lookup is blocked.
As I've already found, Pi-Hole works from the command line, in any case. The only question is getting to the GUI console for ease of configuration and statistics. To get to the admin console, Pi-hole will also work with web admin login page on another port (other than 80) but the errata states that changing from port 80 prevents full web page blocks and that some picture frame blocks will not work properly.
By editing a Pi-Hole config file - lighttpd.conf - to use another port, I can now login to the console with the following. -> omvserverip:8888/admin
In the bottom line, it works but functionality is reduced and the .conf file is over written in a version upgrade. (Much as OMV does in some cases.)
In the bottom line, whether using an Ethernet sub-int and a secondary IP, or some other work around, I believe PI-Hole would make an excellent OMV plugin for reasons previously stated. Advertising is getting way out of hand. While I was testing Pi-Hole I was getting snappy performance from sites with a ton of advertisement. In my experience, this is a first, despite having a cable modem and a fast connection. Again, unless one "likes" being bombarded with unwanted ad's, accidental "one click" redirects, and poor performance from even the fastest connections, Pi-Hole seems to be indispensable.