I am having nearly the exact same issue.
I have many HP Z420 systems I have tried this with. They can all install ANY other OS I have tried via USB in Legacy mode (Non-UEFI). I have built the USB with Rufus, Win32diskImager, dd mode inside of CentOS as well as through Windows, UniversalUSB installer, Unetbootin, Xboot, Etcher, and several other not listed here. It's not the USB drive, as I've used SEVERAL different sticks. I have re-used these sticks after trying Qlustar, to install other OS's, and they still work perfectly fine, and can load the vfat module when installing. Currently, I've built a Rocks7 cluster from these various USB sticks with NO issues. Hell, I've even tried to restore the thumb as a HDD from the qlustar image inside of CentOS to no luck.
The error is simple: run/initramfs/live unknown filesystem type vfat
I've changed the format of the USB to fat16, fat32, NTFS. I've used MBR and GPT partitioning to no avail. I even tried to format the USB as ext2 and ext3, made it bootable, active partition, and then write the iso to the thumbdrive, and still doesn't work (various other reasons as well have occured through these other methods too).
I have searched the internet for many hours to see if I can find a way to fix this. I've tried to exit to rescue disk, and see if i can mount -f or modprobe, but the vfat, or ntfs modules are not loaded to the kernel to allow for this. There has been a suggestion that the order of startup within this kernel is wrong and it's trying to mount the USB drive before it's had a chance to load the module(s) for the USB.
I've also tried this on various sized thumbs... 4G up to 64G... not it.
I've also tried to build a CD/DVD to boot from, and I run into the exact same issue: unknown filesystem type vfat.
It cannot be just you, Jarett. I have no idea where to go from here, as I've tried everything I can find to fix this, other than building this headnode with Ubuntu, then build a new kernel mkinitnfo.... and see if I can get it to hopefully include the vfat module so I can install qlustar.
I might consider trying the Ubuntu build, then maybe, possibly, create a VM inside of Ubuntu, and pray that it can try to install qlustar there, but, it seems unlikely to work, imo.
Any ideas would be great, if someone else knows what might be the issue. I have 1 more non-HP system I can try to install this on, but regardless of the make/model of system, SO FAR, has been unsuccessful.