Timer1 woes on PIC12F629
Figured it out!  I used the system clock (F_osc/4 in the block diagram) instead of the LP oscillator itself since I set the system frequency to 32.768kHz:

T1CONbits.TMR1CS = 0;   // clock source = F_osc/4


I'm not sure if it's a noob mistake to use the LP oscillator for Timer1 when the system clock is also using it, or if I made some other incorrect code/assumption.  I didn't get the fix in the errata (EXAMPLE 1) to work, either, so I'm not claiming complete victory.  But it works!

EDIT: and to quell some anxiety I used 22pF caps on the crystal, too.