Variable valve timing is possible on underhead cam engines, but it's taken years for the mechanisms to be developed and for the benefits to outweigh the costs. The trick is that both intake and exhaust lobes must share one cam, but truly effective variable valve timing requires manipulating the two independently. The solution: concentric camshafts that can rotate a few degrees within each other. The cam lobes that follow the inner shaft then have to slide over the outer shaft with a bushing and link to the inner shaft with a pin.
This is exactly what the 2008 Dodge Viper does, though variable timing is limited to the exhaust lobes. This strategy helps this 600bhp behemoth rank as a low-emissions vehicle, but doesn't yield the earth-shattering increase in output one might hope for.
For that, the timing of the intake lobes needs to be varied as well, by adopting some kind of variable lift system. Fitting a VTEC-style variable lift to a pushrod engine hasn't been done, but it could work. Porsche's Variocam Plus, which mated two cam profiles to a single direct-acting valve bucket, could be mimicked relatively easily, with the pushrod's lifter controlling the same cam-switching action the direct-acting bucket did in the Porsche engine. When the pushrod crew will get around to doing this, however, is anyone's guess.