If you check the History tab of a VM provisioning deployment, you'll see the following tasks:
Submitted
Pre-approval
(any approval levels show up here)
Provisioning
Allocate
Provision
Post-approval
Completed
The IPAM plugin triggered in the Allocate tasks, before Provision begins. This is hard-coded so you don't see any EBS in the GUI.
Just because this happens before your normal EBS subscriptions, you won't be able to modify the input it receives by a workflow. (Actually, there is a way to do that with an approval workflow, but this is a hacky way and has to be enabled on each business group by an entitlement, plus the approval can be switched off by the BG manager). The Resource input does include the hostname (Name field) plus all the custom properties defined at request time.
This hostname is set at request time: either by vRA using the machine prefix, or an action triggered on the form itself.
In our case we generate unique hostname for each VM based on the inputs (function, location, OS) and register the names in a table. Running this action on the request form has a few disadvantages: makes the form slower to respond and we cannot detect if the user hits Cancel, so the hostname remains registered even if it was never used.
So we implemented a different approach: we update the hostname before the provision starts by an EBS (just like you), and when the machine provisioning is completed, we update the hostname in the IPAM system with a REST api call (as the IP is already registered with the vRA machine prefix). We don't use Infoblox, but I'm pretty sure the same is possible with it.
I'm not sure if the Infoblox workflows can be modified, but if you are interested, check the IPAM SDK and you can dump all the inputs vRA sends to the workflow by registering the SDK sample implementation as an IPAM provider.
