A standby/emergency air system is a good idea, one I've thought of doing, never got round to it, but may add it to my SkyGate Hawk. Several real jets I've flown use exactly this system to get the gear down if the hydraulics dump.

For a model its light , simple and easy:

As the leak has occurred in flight the gear is up so the leak is almost certainly in the "UP" lines so the leak there is irrelevant when the "down" side of the valve is opened. So you need an extra air tank (you don't need a CO 2 cylinder but could be used) and the supply line is T'ed into the normal supply line to the gear valve via a Clippard air micro switch. You arrange the servo which actuates the gear valve to overtravel from the "down" position and contact the Clippard valve which opens and supplies standby air to the gear valve. You make the gear servo overtravel by mixing gear to gear and activating the mix via a snap roll button or similar so you dont need a separate servo or radio channel. So, select gear down, nothing happens, then activate the snap button and hold, emergency pressure is fed to the valve and down comes the gear. You don't absolutely need a another air tank, you could arrange the brake tank to be your standby supply.

Of course nothing is guaranteed as the crew of the Polish 767 recently discovered when they landed at Warsaw wheels up after it failed to lower on the standby system ! (If only they had checked ALL the DC circuit breakers !)

Tea finished back to cutting the grass !

Have fun,

David.