It won't be as easy as you might think.
The information about what model each machine on the network is is gathered through Bonjour. Normally, Macs send out a special _device-info._tcp "service" with additional data of, for instance, "model=MacBook3,1". On Linux using avahi, I've been able to set icons with a service entry like this:
Code:
<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
<name>thehostname</name>
<service>
<type>_device-info._tcp</type>
<port>0</port>
<txt-record>model=iMac7,1</txt-record>
</service>
</service-group>
as well as service entries for AFP, SMB, or VNC (doesn't matter, but you need to share one of them for your machine to show up in Finder.)
The second piece of the puzzle is the icons: The icons themselves are in /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources, and the mapping from model identifiers to icons is in /Contents/Info.plist in the same bundle. (Look at UTExportedTypeDeclarations, down toward the later entries. For instance, on my machine, the 17" MBP is #225.)
I think you could export a similar type declaration from any bundle, as long as you were able to get Finder to open it. I'm not sure exactly how this mechanism works; perhaps someone else can chime in with details.
The real question, I suppose, is whether the iPod touch broadcasts a _device-info entry. If not, I'm not sure how you'd make it do so.