If your supply can handle the added current draw, prototype up an excternal regulator.
1)On a small breadboard (PC Proto board) install a femail input connector.
2)Add a output pigtale for the router. (Passive connection to RB500)
3)Tap a line off the +voltage from the female power input.
4)Add a diode in line to isolate the fan noise from the router.
5)Add a capacitor on the far end of the diode to ground 50uf. or so..
5)Add a +12 Volt regulator. LM7812 or similar +12v regulator (3 wires)
Input, Output, ground.
6)Add a 50uf cap to ground on the output (filtering)
The output side of the regulator circuit will be +12 volts and can power a small fan easaly.
It also avoids "tapping" into the RB supply system.
Keep an eye on the regulator, you may need to add a heat sink.
I will not say that the power supply included with the RB500 can handle the added load so keep this in mind, it may affect the ability to add mini pci cards to the system.
DO Not add a fan into the supply of th RB500.
You will most likly inject AC noise onto the power sub system and this will probably create an unstable system, create a poor wireless device or damage the RB power subsystem.
(This is an idea ONLY! Neither MikroTik no I will be responsable for damages to the router or loss of use Etc... if you folow thiese suggestions.)
Drop me a line at
sales@software-routers.com if you need further help.
or elsewhere?
... I test 12V fan, whitch is directly connected to miniUPS connector (is on RB532) only with 100mA fuse at input fan. Fans power take-off is 55,6mA. System working stable more that 24 hours now (1 miniPCI AR5213 card). I will test more time this, but methinks it's fully 
