Programming vintage EPROMs for program storage would be an effort. Adding a ROM monitor and ACIA so that you could use a serial interface to store programs in RAM would be helpful. From there you could bootstrap up to some sort of mass storage. There exist flash-style chips that could be substituted for vintage EPROMs.
The easy part for that is a non-volatile RAM chip (internally, lithium battery, control circuit and CMOS RAM IC). Not sure if those are still around though...
For the trouble, some kind of dual port RAM would be nice. Arduino would just read/write the 68k RAM to load firmware.
But that is the appeal of emulating RAM with the Arduino. It's effectively a dual port RAM, and also a PC interface to make loading it easy.
We used a Dallas Semi part like that in a product. I'm not sure you can still buy them but it would be fairly simple to kluge up a coin cell version today.
My picture of things... battery backed up SRAM for the entire memory space, chips like MC6850 ACIA for I/O.
To load, Arduino pulls 68k reset to tri-state all memory control lines, makes GPIO lines active and executes R/W sequences to access SRAM. Then float all the GPIO, release 68k reset and let it run.
That would do the trick. I would still prefer a MIKBUG-like monitor in nonvolatile memory to load programs though. I'm just used to S-record programming with Motorola parts.
Indeed! I know the difference between 'm' and 'M', it was not my error!
Oooh, yeah! I forgot about the vintage software! I guess if you have an ACIA you can talk directly to the PC, as long as the 68k can find boot firmware somewhere. A 6821 PIA would be a great add on! Or go crazy and add a 6847 VDG for video output!
Nice rabbit hole for 2023!
I still have a source listing somewhere for both a 6809 and a 68k ROM monitor. We manufactured a product with a 6809, a couple of PIAs, some A/D, D/A, IA foo, and a 6847 driving a 5"CRT. Fun memories.
Greatest 8 bit processor of all time. Period.
I couldn't agree more!
Turning things around a bit, if you have a C compiler for 68k or 6809 you could write an Arduino core for it, and upload sketches! 
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Eww. 
Why eww? I didn't mention BASIC.
I had a C compiler for both of those although I preferred Pascal for the 6809, it was the Arduino part I was eww-ing about. 
Well, it would definitely not be period correct.
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Looks like gcc supports the 68000 line of processors https://gcc.gnu.org/onlinedocs/gcc/M680x0-Options.html
I programmed the 68005, 68010 and 68030 processors for about 12 years the were controlling the Unisys high speed check reader/sorters. All assembly language. Lovely processor to work with.
You might look for a personal computer of that age that used a 68005 processor. I had one for a while so see if I could make an emulator for the check sorter. Could not and sold it. I do not remember the name of the device.
If you try to use the 68020 or the 68030, you have to contend with memory management systems.
I still have the code for the check sorters and the 68000 assembler as I did work for the company that bought our company.
Maybe 68008?
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Could be! A long time ago. The chip had an 8 bit wide path to memory and IO, so had to do two read and two write to fill the 16 bit registers. Made for slow processing. The documents are in the box in the loft!
I used a 68008 for a home project so I could use some SS-50 bus memory cards I already had.