Here's the page on installing rEFInd:
http://www.rodsbooks.com/refind/installing.html
Since I doubt the 2012 MacPro is EFIv2, you'll need some manner of a running OS on it in order to do the install - but that could probably be a liveCD of Linux with UEFI support.
Also, I totally get working in a small budget. Just make sure that while IT knows they aren't expected to support your stuff, they should ideally be informed so they don't go "hey, what's this MacPro doing, better unplug it!"
I'm the guy that would look around and say "Hey what's the MacPro doing.." we manage the data center here so they won't unplug anything without checking with us first. So I successfully installed rEFInd selected the boot device that has FreeNAS on it and same error as I listed above, it lets me choose FreeNAS and then Normal Boot and bam...
KDB: debugger backends: ddb
KDB: current backend: ddb
ACPI BIOS Error (bug): A valid RSDP was not found (20170728/tbxfroot-369)
panic: running without device atpic requires a local APIC
cpuid = 0
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xFFFFFFFF82735b90
vpanic() at vpanic+0x186/frame 0xFFFFFFFF82735c10
panic() at panic+0x43/frame 0xFFFFFFFF82735c70
apic_init() at apic_init+0x112/frame 0xFFFFFFFF82735c90
mi_startup() at mi_startup+0x9c/frame 0xFFFFFFFF82735cb0
btext() at btext+0x2c
KDB: enter: panic
[ thred pid 0 tid 0 ]
Stopped at kdb_enter+0x3b: movq $0,kdb_why
db>
I'm going to try it in the standard SATA bay and see if that makes a difference. But rEFInd is detecting the PCIe drive tho...so it's odd.