It is one of the projects I'm working on already some time too. Unfortunately it is on idle mode for a long time. But I can give you some head start information.
A 500mW power mode is in most parts of Europe allowed, hence only in a small defined frequcence range: 869,400–869,650 (5) MHz at 10% duty cycle (which means it is only allowed to send at 10% of the time for your application). Look at page 4 in this extract of the Official Journal of the European Union for more information.
So, as far as you tune the module to the right frequency (and you do not send the whole time) you don't break any laws. The tuning is done in the RF12.cpp (rf12_initilize). I once calculated the correct values with the RF12 documentation at hand, but apparently lost them again.
To the hardware part: The RF12BP needs two different power levels, the usual 3,3V and for the amplifier dedicated 12V. So it makes most sense to drive the JeeNode with stable 12V power source. This way we can connect the RF12BP directly to the PWR pin anywhere.
My approach was to build an RF12BP Plug. As of this I connected the the two power sources, ground and the interrupt of one Port. For the SPI I did attach some wires.
You see that I also incorporated a barrel plug for power. This is necessary as it is not really possible to drive the RFM12BP with batteries.
As already told I never found the time to try the whole setup. But if it is build as described here, you have to make an other small change in the code. The interrupt has to be changed to the ports interrupt.
Further I already bought a 868 MHz antenna what should extend the range even further. I think transmit ranges around 10km should be possible.
Please inform me if someone makes progress in this project.