Just in case anyone else is having problems... I couldn't get the sysclk_driver example code to start-up an external crystal oscillator. I didn't try to track down where the problem is (driver or includes or me using the driver).
Anyway, the following works to get a 7.3728Mhz crystal running with a system clock of 29.4912Mhz:
cli();
// 2-9 Mhz, 256 clk
//0 1 0 0 0 0 1 1
OSC.XOSCCTRL = 0x43;
//enable external oscillator
OSC.CTRL = (1<<3);
//wait for stable
while( ! (OSC.STATUS & (1<<3)) );
CLKSYS_Prescalers_Config( CLK_PSADIV_1_gc, CLK_PSBCDIV_1_1_gc );
//select external osc
CLK.CTRL = 0x03;
sei();
//scale up by 4x for 29.4912Mhz
CLKSYS_PLL_Config( OSC_PLLSRC_XOSC_gc, 4 );
CLKSYS_Enable( OSC_PLLEN_bm );
do {} while ( CLKSYS_IsReady( OSC_PLLRDY_bm ) == 0 );
CLKSYS_Main_ClockSource_Select( CLK_SCLKSEL_PLL_gc );
