My first thought was this:
https://dmmcheckplus.com/shop/ols/products/dmm-check-plus-fully-loaded-with-all-options-lc-board-enclosure-and-dual-frequencyBut given the cost and the equipment you have, I am not sure if it makes a ton of sense to spend that kind of money.
If not, maybe an option could be to go the route of sanity cross-checking combined with multiple DMMs and other test equipment.
Different ranges and functions use different paths/configurations. Different resistor dividers, offset and gain calibrations for each individual range/function, etc.
So apply 3V to the Fluke 45 and Hioki, then manually range up to 30V, 300V, 1000V, and see if everything is consistent. In some devices it's worth reversing the leads. The Fluke 45 seems to have multiple negative DC V performance tests but only a single -90mV calibration point. If you look at the theory of operation of some DMMs, this method isn't always overly useful if the same passives are used in multiple ranges and they've drifted. But if things align, then that provides some level of confidence I'd think.
You could get some 0.1% resistors from DigiKey/Mouser and use them with Ohm's law. I picked up a pile some time ago and perhaps since they are designed to hold that spec through various temperature ranges, I found that at room temperature they were typically far better than spec. So a 1K 0.1% resistor is probably going to easily be between 999.5K and 1000.5K. Cross check the voltage drop with the current flow and resistor value and see if it all lines up.
If it DOESN'T line up, then you have more work to do, but I suspect things will come out pretty good given your test equipment's pedigree.