1680x1050 capable Nubus and/or LC PDS cards?

No,and no (windescreen support came way after 68K macs).

I wonder if some ReEdit Kung-Fu might work here. There were portrait monitors that had a non standard aspect ratio. So it must be possible for it to work. I would think that it would be possible with one of the high end NuBus video cards.

Just about any video card, even the onboard video in the LCIII and Centris 650 will run a VGA LCD with a Mac -> VGA adaptor, but only at "normal" resolutions (640x480, 800x600, 832x624, 1024x768, 1152x870). So it will work, it just won't look right, due to the fact that the Mac will be running at 4:3.

The Supermac Thunder series could do 1600x1200, thats as high as I have seen.

Portrait monitors had special controller cards didn't they? Also LCD can't do sync on green can they?

Good point on the SoG. Some LCD monitors do support SoG, though I it think it is mostly the higher end displays.

I think the portrait displays just use a Mac display connection. According to the manual for my IIsi the onboard video supports the 15" Apple Portrait display.

Yeah, the Portrait Displays use a special cable that has a regular Mac DB-15 plug on one end for the Mac, and an I3W3 plug on the other end for the display.

There was also a Nubus card for the Portrait monitor with an I3W3 connector, which required a cable with the same connector at both ends. There has been one of these Nubus cards on eBay of late, but without the cable.