What you are looking for is EXACTLY what I was looking for a while back before I bought a 340mb microdrive. I have the microdrive in the IA-1 now, and it runs windows 98 okay. Got the linksys usb 10bt connected to it and can use it just like a normal computer albeit a fair amount slower. However, I still have some things in the device manager to work out - some things showing up wrong. Sound works, haven't tried the modem (have a driver though), but it doesn't like the cyberblade video driver, keeps wanting to use the trident super vga.
Installing windows on this was much more difficult than on a regular PC due to space limtations, and the ia-1 hanging on bootup with all the ACPI stuff loaded.
I would certainly prefer to have a bigger HD connected to the IPAQ but I was unable to find any of the adapters that you described. On the compact flash, I have jailbait running with the most important thing not yet working - the linksys 10bt - it needs a different usb module.
If you come across that adapter, let me know, as I would like to have one. But then where you we get the power source for the hard drive? Good luck finding the adapter!