The afp.service file allows adding a "model" identifier string, this can be looked up on a Mac in /System/Library/CoreServices/CoreTypes.bundle/Contents/Info.plist. The model identifier will make the device show a different icon based on the configured string.

Some valid codes are:

Windows
Windows2000
WindowsXP
Windows2003
WindowsVista
Windows2008
Macintosh
PowerBook
PowerMac
RackMac
Xserve
Macmini
iMac
MacBook
MacBookPro
MacBookAir
MacPro
AppleTV
iPhone
iPod
iPad
AirPort
TimeCapsule

There are many more codes available in /System/Library/CoreServices/CoreTypes.bundle/Contents/Info.plist. To get them, convert the plist to XML using plutil:

plutil -convert xml1 Info.plist

and then load the plist with a regular text editor.

afp.service

<?xml version="1.0" standalone='no'?>
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
  <!-- Customize this to get a different name for your server in the Finder. -->
  <name replace-wildcards="yes">%h</name>
  <service>
    <type>_device-info._tcp</type>
    <port>0</port>
    <!-- Customize this to get a different icon in the Finder. -->
    <txt-record>model=RackMac</txt-record>
  </service>
  <service>
    <type>_afpovertcp._tcp</type>
    <port>548</port>
  </service>
</service-group>