can I use a Miller Solar engine for RF applications?
The 'Miller Solar Engine' is a device for turning on and off a heavy load, like a motor or lamp, in solar powered toys. Part of the fun is that you don't know when the capacitor will be charged enough to give you a few seconds of movement or light. I'm sure the amusement wears fairly thin after the 307th burst of activity. 

A supercap cannot be over-volted without damage. The only way the 'MSE' attempts to control the maximum voltage is to turn the load on at the threshhold voltage. From the descriptions, the threshhold is 2.7v, which will be too high for some makes of supercap. If the load cannot absorb the excess current from the solar panel, then the capacitor voltage will continue to rise anyway.

This is not what you want for a low power radio. With its low input current when the input power supply is removed, the 3225 looks a better bet, assuming the panel output voltage is compatible.

When dealing with a solar powered link, you have to be aware of the need to do an orderly shutdown (depending on the link protocol) when power vanishes unexpectedly. Having supercaps mitigates this to some extent by giving you a minimum time for that, once the panel power has disappeared. You would need to sense the panel voltage separately to the capacitor voltage to be able to make use of this warning.