I am not sitting in front of that machine right now, but yes I believe "Open ROM" is the wrong choice. Check the menus for something that offers to install a ROM into Multi Flasher. Once you do it you'll get two file pickers, and maybe also a warning dialog.
The Graphiccelerator UI is confusing. You need to pick a menu item to install the ROM, not open multiflasher. So double check that you’re actually getting the rom loaded into Multiflashsr
I have flashed two with Multiflasher on a G5 running Mac OS X. One a 32 MB PC card that already had a 128K rom chip, another one of those 64 MB ones that are on ebay for $25 or so. I had to put a bigger ROM chip on that last one, I got the part number from bbraun's radeon 7000 page.
I never had much success with the reduced ROMs. I bought some 128K rom chips and successfully replaced one on a 64 MB radeon 7000 a few weeks ago, and flashed the retail 226 ROM on it. I'm not running Mac OS X though. Looks just like the silver heatsink ones on the left side of your picture.
There is a very basic MacRoman <-> UTF-8 converter in one of my Marathon tools: https://github.com/treellama/atque/blob/master/ferro/macroman.cpp
If your project is GPL you can use that directly. Depends on how much modern C++ support you have too, I suppose.
If one or the other of those is...
In my case it was not an AppleTalk or AP issue, as I was only trying to use IP at the time. But, I could have been doing something wrong. If you get the Ethernet working, should be straightforward to switch the interface to the Wifi and try it!
I have had trouble getting bridge to work with the Raspberry Pi wifi, so YMMV. If you have trouble with it, try Ethernet before going too far debugging it.
I now have tashrouter working as well. The writeup by @finkmac was helpful. In my case, netatalk is running in a VM. I was able to run tashrouter in a separate VM, but with libvirt getting one to start up before the other was tricky. So I ended up using the tap / bridge setup above directly on...
My pre-FW Clamshell (to my surprise, at the time) boots from a USB stick with 9.x on it. This is a relief as the CD-ROM barely functions so I have no other way to boot it without setting up a net boot server.
I have not been able to get it to boot Mac OS X from USB however.