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Allow me to provide two suggestions that don't answer your direct question, but go towards your root concern:

• Have you considered virtualizing OPNsense?  You might have more luck managing the power of the base system this way.  While Proxmox, like OPNSense, is also an appliance wrapped around an OS distro (in this case Debian instead of FreeBSD) that ideally you'd leave more or less alone, I feel its underpinnings are more tinkerable (within reason) without setting yourself up for maintenance issues down the road.  Also, just the fact that it's Linux-based and not Unix may provide more customization options (the fact that you are here looking to install a FreeBSD port of a Linux driver makes for a good example) and broader community support.
  
  Yes, it's another layer of abstraction, but that can work to your benefit here.  As @loonylion mentioned regarding "N-series appliances":
  
  

  bios options vary wildly between systems and even bios versions

  
  When the day comes to upgrade your base system or replace it due to a failure or what have you, you may be hard-pressed to find one whose power management idiosyncrasies are the same as before.  Again, I'd rather deal with Linux than Unix here.
  
  Admittedly, there are security implications to consider when virtualizing a firewall that I won't wander too far into here, but enabling PCI-passthrough on the hypervisor for at least the NIC corresponding to the WAN port goes a long way.
  
  
• Consider investing in a power monitoring device of some kind, be it a simple, in-line meter or a consumer-grade smart power strip (if you search for "smart PDUs" you'll get more enterprise-class gear that is great for its intended purpose, but gets very expensive).  You mentioned:
  
  
  

  anecdotal evidence on the net suggests up to 3W, which on my N-series appliances could be as much as a third of the total power draw

  
  If you're trying to find ways to reduce power consumption you really need to be able to measure the results of your efforts--specifically at the wall. I have a Kasa Smart Plug Power Strip HS300, but I couldn't find a UK version.
  
  

I run a Proxmox cluster of three passively-cooled mini PCs (two N100s and an N5150) in an expensive power market and have been working on setting up OPNsense on two them to run in an HA configuration to replace a bare metal pfSense device.  Inspired by your question, I did a quick Google search for "proxmox manage igpu power state on n100" and came across this suggestion for how to toggle Proxmox's CPU frequency scaling governor.  I tried it and instantly saw a full 1W drop in power consumption.  Granted, this is just a preliminary finding on what was a mostly idle system and I don't yet know what the performance hit will be, but the point is that Proxmox gave me options and I was able to measure the actual result of the intervention.