For a stock-suspension '07 S2000 enthusiastically street-driven - but not tracked - with just a driver (i.e., no passenger), is there any consensus about getting the alignment done with the driver's weight in his seat?? My reasoning is that it can't hurt, but does it really make any difference?

 

Give these a read:
https://robrobinette.com/corner_weight.htm
https://robrobinette.com/S2000Alignment.htm

On a street-driven s2000, I doubt you'd notice a difference. On the other hand, why not wait in the cockpit instead of the shop's grungy waiting room.

 

Because humans suck at holding still, so when the alignment guy tells you to hold the wheel so he can center your steering you'll do it wrong. Then you'll come back and bitch about a crooked steering wheel. Then I have to put your car back on the lift go through the motions again because you can't live with the wheel 3 degrees to the right.

 

Health and safety issue.
Not allowed in the car in the shop on ramps.

 

yes it makes a difference.

 

I use to race Spec Miata and that is how we do not only the alignment but the corner balance on the suspension as well.