I found a blogpost describing the setup of the icon:
http://www.simonwheatley.co.uk/2008/04/06/avahi-finder-icons/
So create (if not already present) a file:
/usr/local/etc/avahi/services/afp.service
And put e.g. this in it:
<?xml version=”1.0″ standalone=’no’?><!–*-nxml-*–>
<!DOCTYPE service-group SYSTEM “avahi-service.dtd”>
<service-group>
<name replace-wildcards=”yes”>%h</name>
<service>
<type>_afpovertcp._tcp</type>
<port>548</port>
</service>
<service>
<type>_device-info._tcp</type>
<port>0</port>
<txt-record>model=Xserve</txt-record>
</service>
</service-group>
This will display the AFP server as an Xserve 
The different model found in the plist that defines the graphics are:
(/System/Library/CoreServices/CoreTypes.bundle/Contents/Info.plist)
| Name | ID | Models | |
| PC | public.generic-pc | Windows Windows2000 WindowsXP Windows2003 WindowsVista Windows2008 Windows7 |  |
| Mac | com.apple.mac | Macintosh |  |
| Laptop | com.apple.mac.laptop | Laptop |  |
| Tower | com.apple.mac.tower | Tower |  |
| PowerBook | com.apple.powerbook | PowerBook |  |
| PowerBook G4 | com.apple.powerbook-g4-titanium | PowerBook3,2 PowerBook3,3 PowerBook3,4 PowerBook3,5 | |
| PowerBook G4 | com.apple.powerbook-g4-12 | PowerBook6,1 PowerBook6,2 PowerBook6,4 PowerBook6,8 |  |
| PowerBook G4 | com.apple.powerbook-g4-15 | PowerBook5,2 PowerBook5,4 PowerBook5,6 PowerBook5,8 |  |
| PowerBook G4 | com.apple.powerbook-g4-17 | PowerBook5,1 PowerBook5,3 PowerBook5,5 PowerBook5,7 PowerBook5,9 |  |
| iBook G4 | com.apple.ibook-g4 | PowerBook6,3 PowerBook6,5 PowerBook6,7 |  |
| Power Mac | com.apple.powermac | PowerMac |  |
| Power Mac G4 | com.apple.powermac-g4-quicksilver | PowerMac3,5 |  |
| Power Mac G4 | com.apple.powermac-g4-mirrored-drive-doors | PowerMac3,6 |  |
| Power Mac G5 | com.apple.powermac-g5 | PowerMac7,2 PowerMac7,3 PowerMac9,1 PowerMac11,2 PowerMac11,2,Quad |  |
| Xserve | com.apple.xserve | RackMac Xserve |  |
| (null) | com.apple.macmini-unibody | Macmini4,1 |  |
| (null) | com.apple.macmini-unibody-no-optical | Macmini Macmini5,1 Macmini5,2 Macmini5,3 |  |
| eMac | com.apple.emac | PowerMac4,4 PowerMac6,4 |  |
| iMac | com.apple.imac | iMac |  |
| (null) | com.apple.imac-g4-15 | PowerMac4,2 |  |
| (null) | com.apple.imac-g4-17 | PowerMac4,5 PowerMac6,1 |  |
| (null) | com.apple.imac-g4-20 | PowerMac6,3 |  |
| iMac G5 | com.apple.imac-g5 | PowerMac8,1 PowerMac8,2 |  |
| (null) | com.apple.imac-g5-isight | PowerMac12,1 |  |
| (null) | com.apple.imac-core-duo | iMac4,1 iMac4,2 |  |
| (null) | com.apple.imac-core-2-duo | iMac5,1 iMac5,2 | |
| (null) | com.apple.imac-iSight-24 | iMac6,1 | |
| (null) | com.apple.imac-aluminum-20 | iMac7,1 iMac8,1 |  |
| (null) | com.apple.imac-aluminum-24 | iMac9,1 |  |
| (null) | com.apple.imac-unibody-21 | iMac11,2 iMac12,1 |  |
| (null) | com.apple.imac-unibody | iMac10,1 iMac11,1 iMac11,3 iMac12,2 | |
| MacBook | com.apple.macbook | MacBook |  |
| (null) | com.apple.macbook-white | MacBook1,1 MacBook2,1 MacBook3,1 MacBook4,1 MacBook5,2 MacBook1,1,White MacBook2,1,White MacBook3,1,White MacBook4,1,White |  |
| (null) | com.apple.macbook-black | MacBook1,1,Black MacBook2,1,Black MacBook3,1,Black MacBook4,1,Black |  |
| (null) | com.apple.macbook-unibody | MacBook5,1 |  |
| (null) | com.apple.macbook-unibody-plastic | MacBook6,1 MacBook7,1 | |
| (null) | com.apple.macbookpro-13-unibody | MacBookPro5,5 MacBookPro7,1 MacBookPro8,1 |  |
| MacBook Pro | com.apple.macbookpro | MacBookPro |  |
| (null) | com.apple.macbookpro-15 | MacBookPro1,1 MacBookPro2,2 MacBookPro3,1 MacBookPro4,1 | |
| (null) | com.apple.macbookpro-15-unibody | MacBookPro5,1 MacBookPro5,3 MacBookPro5,4 MacBookPro8,2 | |
| (null) | com.apple.macbookpro-17-unibody | MacBookPro5,2 MacBookPro6,1 MacBookPro8,3 |  |
| (null) | com.apple.macbookpro-17 | MacBookPro1,2 MacBookPro2,1 |  |
| MacBook Air | com.apple.macbookair | MacBookAir1,1 MacBookAir2,1 |  |
| (null) | com.apple.macbookair-11-unibody | MacBookAir3,1 MacBookAir4,1 |  |
| (null) | com.apple.macbookair-13-unibody | MacBookAir MacBookAir3,2 MacBookAir4,2 |  |
| Mac Pro | com.apple.macpro | MacPro MacPro1,1 MacPro2,1 MacPro3,1 MacPro4,1 MacPro5,1 | |
| Apple TV | com.apple.apple-tv | AppleTV AppleTV1,1 |  |
| iPhone | com.apple.iphone | M68AP iPhone1,1 |  |
| iPhone 3G | com.apple.iphone-3g | N82AP iPhone1,2 |  |
| iPhone 3GS | com.apple.iphone-3gs | N88AP iPhone2,1 | |
| iPhone 4 | com.apple.iphone-4 | N90AP iPhone iPhone3,1 |  |
| iPod | com.apple.ipod | iPod |  |
| iPod touch | com.apple.ipod-touch | N45AP iPod1,1 |  |
| iPod touch | com.apple.ipod-touch-2 | N72AP iPod2,1 |  |
| iPod touch | com.apple.ipod-touch-3 | N18AP iPod3,1 | |
| iPod touch | com.apple.ipod-touch-4 | N81AP iPod4,1 | |
| iPad | com.apple.ipad | K48AP iPad iPad1,1 |  |
| AirPort Express | com.apple.airport-express | AirPort4 AirPort4,102 AirPort4,107 |  |
| AirPort Extreme | com.apple.airport | AirPort AirPort5 AirPort5,104 AirPort5,105 AirPort5,108 AirPort5,114 AirPort5,117 |  |
| Time Capsule | com.apple.time-capsule | AirPort6 AirPort6,106 TimeCapsule TimeCapsule6 TimeCapsule6,106 TimeCapsule6,109 TimeCapsule6,113 TimeCapsule6,116 |  |
Facebook
Google+
LinkedIn
