Does LV sampling work when ..say.. a 120 second image is being captured?

I don't know of a way to do that. I don't think the cam will ever display an image on LV while a capture is in progress. The readout process from the sensor is fundamentally decoupled from the capture. You could probably interleave long exposures with short ones at greatly boosted ISO, and display only the short ones on LV.

I was assuming it would be possible to quite accurately model the drift over time, and adjust the model based on the last image. The model continuously guides the mount, and the lag in updates hopefully wouldn't matter - so you can use saved images, not LV. In fact, we can trigger actions to occur on the in memory image just before writing out.