Last night, I put together a shell script that allows you to trick the iPhone into thinking that amr files added to the voicemail folder are actual voicemails. To make this happen, I ported sqlite3 to the iPhone (You can download a copy here). I also wrote a csh script, which you can download here and a time utility, here. The reason I wrote the script in csh rather than bash (both of which appear in the standard binary distribution kit) is just that I'm more familiar with csh.
To run the script, supply it with the amr file as its one argument, e.g.
copy2vmail foo.amr. The script copies the amr file to the voicemail folder and updates the voicemail database as if the voicemail were received at the current time.
To force voicemail to update and re-read the database, enter the phone application and quit it by holding down the home button for 4-8 seconds. Re-enter the phone application and, with luck, you will see the recording as new voice message from "VoiceRecorder".