Yeah, I don't think it will work that way. A simple test with 3 OSX Macs is from your first Mac, mount a second Mac. Then using a third Mac, mount the first Mac. You will note you are not presented with the option to mount the second Mac's drive, regardless of the sharing settings for both Macs.

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I've been working on this for a few days. Found some interesting things out. In general, as you pointed out, shared drives cannot be shared again to another Mac. I did find some interesting exceptions, though. If I mount the public folder of someone's iDisk on my iMac, my Color Classic can see it and connect to it. Same goes for a FTP share created from NetPresenz. Neither of these can be shared from the OS or from Sharepoint. But if I log into my iMac from the Color Classic with my owner account, I can connect to these shares. What's weird is, iDisk uses WebDav, but if I make my own WebDav folder, I can't share it forward. The only thing the two different shares (iDisk and FTP) had in common was that they were read only.

There seems to be ways to make it work. I'm going to keep at it. If nothing else, I could mount a FTP share from a Mac Plus/SE/Classic to my iMac. It will show up as a drive, and from there the iPhone can access it. Still leaves the 512k out though....