So, I figured out how to do this, and as requested, I'll try to provide a synopsis of what I did.
First, to explain my configuration and what I was trying to accomplish. I have an ASUS M2V HDMI motherboard, which has 4 onboard SATA ports, and a 4 core AMD Phenom CPU, and 8GB of RAM. I did put an Intel GBE card in it when I had it running ESXi 3.5, since the onboard RealTek was an impossibility.
Enter ESX4i. I wanted to do the upgrade, and found that the entire layout of the install image had changed from what I knew in 3.5i. Gone was the oem.tgz and all things familiar, so I set off trying to figure this all out. I found a few pages that were VERY helpful.
I began by reading the ESX4i version of "Customizing your ESXi install with oem.tgz" that was posted here. (Thanks Dave!)
This was the foundation for what I then undertook. I started by grabbing one of the pre-existing oem.tgz files referenced in the above link (I started with the 3Ware RAID controller '3ware.esxi4.oem.tgz' file). I then followed this process to create a bootable USB with the ESX4i ISO on it. I started with a FAT16 1GB USB drive, and followed steps 1-3 verbatim. For step 4, I had to modify the isolinux.cfg to add the oem.tgz after the ienviron.tgz and before image.tgz entries (not sure if another order would have worked, but this seemed to jive with the order that had been used in 3.5i, and it worked the first time, so I didn't tempt fate!). I then subsequently copied the modified 'isolinux.cfg' to the 'syslinux.cfg'. Step 5 was as documented. For step 6, I substituted "dd if=mbr.bin of=/dev/sdc"rather than using 'cat', as I'd had better success with that when I did the same thing for ESX3.5i. Step 7 needed to be tweaked, at least for me. I had to specify the partition number when running syslinux (i.e. "syslinux /dev/sdc1"), otherwise you'd get a "syslinux: this doesn't look like a valid FAT filesystem" error.
The next step is to modify the oem.tgz file to suit your needs. In my case, I had to remove some other items that had been referenced in the basline oem.tgz I used, as my hardware IDs were in conflict. I extracted the oem.tgz file, and modified the 'simple.map' file to add my bits ("1002:438c 0000:0000 storage ahci.o"). I then modified the 'pci.ids', removing a conflicting entry that was already there, and replacing with what had worked in 3.5i ("438c ATI Technologies Inc Class 0101"). I then removed the various object (.o) files that I didn't need for my use, and recreated the oem.tgz file. I then copied this oem.tgz to the root of my USB drive.
The final step that I took was to rebuild the image.tgz file, to include the necessary oem.tgz into it. This process works flawlessly, and I followed it pretty closely, simply replacing the oem.tgz file inside the .dd file with the one I created just above. I then rebuilt the image.tgz file, and copied it over the one on the USB key.
I shut down all my VM's that were running on the 3.5i server, and then booted the USB key, and installed. Be aware, if you have any datastores on your target boot disk, those will get zapped, but luckily I only had ISO's on that datastore, and they were easily copied back to the system. After installation, ESX4i discovered my existing datastore where my VM's lived, and even handled the striped configuration I'd set up just fine. I then had to connect to the ESX4i system using the vSphere Client, browse the VM datastore, and then "add to inventory" all of the .VMX files that I cared about.
So, I'm pleased to say that I answered my own question, with a bit of work and leveraging resources I'd found for the 3.5i install, where I had to do similarly. Public thanks to Dave Mishchenko and Cameron for the reference material I was able to leverage!
I sincerely hope that this process will be beneficial to someone, besides me. It wasn't as hard as I feared, and allowed me to upgrade pretty easily. I'm also attaching my resulting oem.tgz file (with the above entries in it), as well as the isolinux.cfg file that I used.
