I'd be interested in seeing/reading this too.
I've done similar on my Netbook but not at the exclusion of the standard desktop.
My netbook boots Xubuntu with a custom AmigaOS background for the greeter (login screen). If I log in as my normal user I'm presented with the standard XFCE desktop. However, if I log in as 'amiga', XFCE starts but instantly spawns FS-UAE (from a user perspective XFCE is invisible unless FS-UAE is minimised). FS-UAE boots to a high res Picasso OS3.1 display, showing the same AmigaOS wallpaper as the greeter/login screen. It *looks* like I've simply logged into AmigaOS
It's great for baffling other commuters!
Your solution seems much better for a pure Amiga experience though!