I am not in a position to even attempt this, so I will post it here.
Here is what I propose in the loosest terms. Once ore is registered, upon world gen to have it be thus so that instead of calling for your mods ore block, To allow you to call the forge ore dictionary to acquire a replacement block under the same oredict entry.
This would mean that all mods could generate the same ore by using the ore dictionary to dictate what block to generate.
You could have it by default use the first ore registered of that type and generate a forge config entry to change it if the end user so desired.
I don't use forge and I never touched java, I prefer other languages. Where mine craft is involved I am just a end user with a dirt house. however I gather that I might just be possible for this modification to forge to work.