I haven't played with the 328P much, but according to the data sheet, section 9.10:
When using the Timer/Counter Oscillator, the system clock needs to be four times the
oscillator frequency. Due to this and the pin sharing, the Timer/Counter Oscillator can only be used when the
Calibrated Internal RC Oscillator is selected as system clock source.
So I suppose you could replace the resonator with a 32.768 kHz crystal, use the internal 8 MHz RC oscillator for the main clock (with suitable fuse changes), and use the 32.768 kHz crystal for Timer 2. But I haven't tried it so I don't know if there are any issues you would run into.
I attached a small lib I've started to manage timer 2. It is very much a work in progress, and probably requires some #ifdef blocks to select the correct bits for 328p, but it may be useful to you.