Ha, I'd tried driving my Portrait display with the DOS card in my 6100 back in college. Out of the box, I obtained the result you've shown.
So I thought, "I know what I'll do" which was to install a Unix-alike on the DOS card that could run XFree86. In those days, this variant of the X Window System was configured by a scary text file called
XF86Config, which required the user to specify
low-level monitor details like vertical and horizontal refresh rates. You had to edit this file by hand (and then pray) to get graphics! I thought, "Sure, this is a scary game, but this file gives me the power and expressiveness I need to get the video system on the DOS card to drive the Portrait display."
Now I can hear what you're saying: "Hey
@stepleton, even back in the day, Linux wanted to talk to the hard drive hardware directly, and that doesn't work since the DOS card only lets you read and write to the 'hard drive' (disk image file) through BIOS calls!" To which I'd say "sure, but who said anything about Linux?
MINIX is a UNIX-ish OS that's happy to get by with BIOS calls, or at least it was back then."
After a moment you say "But come now
@stepleton, there was only MINIX 2 while you were in college and that version of MINIX couldn't support XFree86 or any other X variant!" To which I'd say "That is true, but you forget there was also
Minix-vmd, which DID do XFree86, that's part of what it was for." And that's what I run on my "DOS" card even today; it works just fine.
So what about the Portrait display? Well, I messed around a little bit and made the monitor flash a bit, but was never fully successful. Not in a "it can't work" way but more in a "I'll get back to this later" way. I went back to the regular colour CRT... and now well more than 20 years have passed. I've been meaning to get around to it, and I even have a Portrait display to try it with --- but I'll need to recap it first.
If someone else would like to get the Portrait display to work with their DOS card, my first recommendation is to try XF86Config hacking under Minix-vmd. (IIRC the current MINIX generation --- MINIX 3 --- dropped support for BIOS-based HD access, but you should check.) I think there's still a chance this approach could work, and it may be easier and even more useful than writing your own program that talks directly to the video system under DOS or whatever. Good luck!