There is some gauge / Autopilot that allows to very low flyig - following terrain? You know, up and down the hills... I know many modern figters / bomber have TFR to evade enemy radars...
Many FS models have the radio altimeter that basicaly indicate the heigh over the terrain... so I think is posible, to keep a constant heigh like the ALT autopilot but link to the Radio Altimeter.
Ah, for FS2004 please..