1) The firmware with the secret sauce is version MM71.0226.B00 (under Windows 10, this is reported as MM71.88Z.0226.B00.1709290808 instead) with a date stamp of 9/29/2017.
Once the firmware of your Macmini7,1 is upgraded to this version, Intel graphics drivers will just magically begin to work with Win10 1703 (Creators) and 1709 (Fall Creators).
If the firmware of your Macmini7,1 has not been upgraded to this version, Intel graphics drivers will continue to fail to install/upgrade with Win10 1703 (Creators) and 1709 (Fall Creators), just as they have since 1703 was released.
It's that simple. Check the Boot ROM Version under System Information on the macOS side, and if is not yet at MM71.0226.B00, there's no point in trying to install/upgrade Creators/Fall Creators until you do have the Boot ROM at that version.
2) Side note: the Install macOS High Sierra.app you can download from the App Store is still plain old 10.13 — not 10.13.1beta — even if you have the macOSDeveloperBetaAccessUtility.pkg installed. This was a rookie mistake I made, thinking that installing macOS Beta Access meant I would get a special version of the Install app that would let me upgrade directly from 10.12.6 to 10.13.1beta — not so. There is a firmware update in the current Install macOS High Sierra.app, but it is only MM71.0224.B00, which does not contain the necessary Intel graphics fix.
3) The MM71.0226.B00 firmware upgrade will come down and install the first time you run Software Updates on 10.13 with macOS Beta Access installed. Once that software update is complete, you're good to go.
Another side note: when the High Sierra Beta Update has finished downloading, if you prevent it from restarting your Mac to perform the installation, the firmware update package file can be found under the /Library/Updates directory (i.e. for 10.13.1 beta 5, the location is /Library/Updates/091-41339/FirmwareUpdate.pkg). This FirmwareUpdate.pkg can not be installed on its own as-is, but enterprising administrators could unpack it with Pacifist or Munki and easily make a standalone installer for the MM71.0226.B00 firmware upgrade. I made one of these, and it works fine, which leads me to...
4) Apart from the MM71.0226.B00 firmware upgrade itself, there's nothing magical about any version of macOS that affects the Win10 Creators Intel graphics issue. Once you have the firmware updated, you could even wipe the entire drive and reinstall ancient OS X 10.10 if you wanted, and installing Win10 Creators would still work properly. I don't know why you'd want to do that, but my point is that you could do it. This fact was actually important in my environment, because I have more than 70 of these Mac minis that need to stay stuck at macOS 10.12.6 until next June for application compatibility reasons, but we desperately need to update their Windows 10 OS to Creators before then because Win10 1607 will reach End Of Life support in March. After I run my standalone MM71.0226.B00 firmware updater on them, they can upgrade to Creators or Fall Creators without having to change anything about the macOS installation. Yay!