Oh, yes, most folks with desk jobs would still be working -- at least I sure was. My company's pregnancy leave is day-limited; if you leave before delivery you have to dock those days off your maternity leave.
With SPD, being seated upright in a chair is the least painful position possible. Standing is excruciating, but lying down is nearly as bad, because you cannot move your legs or hips without pain, but you always want to in order to shift the weight of the baby. SPD isn't dangerous in terms of one's systemic health at all; it is just pure pain, coupled with the risk that one of your legs will fold under you if you can't manage to hold your hip joints stiffly enough whilst walking.
With my youngest, my OB finally told me that I couldn't go to work any more at my weekly visit of week 39; that was on Wednesday. I delivered on Saturday, and yep, I had to go back to work at 6 weeks minus 3 days because of it. DH actually got laid off during the final week of my leave, which is what saved us; our daycare would not take an infant younger than 6 weeks.
I was essentially fine as long as I was sitting at my desk with my legs propped up, but I tried hard to stay in that chair and not stand unless it was absolutely necessary. In each of my pregnancies I was using a cane by 15 weeks.
Personally, I wouldn't have needed company-issued "medical parking" either -- because my OB always signed the paperwork for me to obtain a DMV temporary disability placard until the end of my due month. However, there are a lot of OB's (mostly males, of course) who dismiss SPD as imaginary or just not that painful; if I had had a physician like that and medical parking was available, you bet I would have applied for it.