If you're using two oscillators, you could simply modulate them in opposite directions (i.e. one positive and the other negative) using the same LFO set to a very slow rate and a depth of just 1. That would create the elusion of drift without upsetting the perceived center of pitch. Using two LFO's set to different rates works even better in this regard. But if you're after yet more subtlety, that's what the 'Drift' parameter is for, no? And if you're really looking to split hairs, detuning a single oscillator by just 1 cent using the Fine Tune parameter, with the slop also engaged, will definitely find you some middle ground while still being random enough. For that matter, heavy but slow pulse-width modulation also creates a detune effect of sorts. Etc.
Anything more subtle than that and I have to ask the rhetorical question, what's the point?
Cheers!