Measuring stress from pulseox variability?

Yes, you can do it - the standard heartbeat detector that's built in doesn't do this since it uses a autocorrelation approach, but you can still access the raw data and figure it out from there - although the data is very noisy whenever you move your hand so you'd have to put a lot of effort in to filter the data.

Of course you can get a decent idea very easily just by comparing the BPM values you'll get from the built in detector every few seconds and seeing if they change noticeably.