Hi Felix,
The general rule of thumb for the tunability/pulling of a crystal is that --depending on the activity of it and the chosen oscillator circuit-- it can be tuned by a few hundred Hertz for every MHz, usually with a series capacitor which can be partly or fully the recommended loading capacitor. This means for instance that if you have a 1MHz crystal, then you can tune it by 400-700Hz, maybe 1kHz without losing much stability. This is bad news for your 32.768kHz crystal, because this rule gives some 10-30Hz or so tuning possibility...
Nevertheless, you may test it by building your oscillator with CMOS gates like the one at h**p://www.ee.washington.edu/circuit_archive/circuits/F_ASCII_Schem.html#ASCIISCHEM_008 and you may replace the 39pf capacitor first with a variable trimmer to experience your crystals' pulling ability/activity. I mean the circuit with the title: "Crystal 32.768KHz CMOS Oscillator". If you are satisfied with the pullability, then you may replace the variable cap with a similar value varicap diode and see how you can tune with that.
So with your test circuit you could gain useful practical experience on your crystals.
The reason why I recommended a higher crystal frequency earlier was that the pullability increases with frequency and everything gets easier in this respect but in your case if the current consumption counts so much, then you are left with the harder way...
Good luck,
unkarc