Well, the Digital Mars C/C++ compiler still supports DOS programming (I just can't seem to let that go!)

Good to know!

I thought that OpenWatcom was the only one remaining to support DOS (and it appears to have been stagnant for a couple of years)...

(Not that I've done much with DOS - I did write a ~10-20 line program in C to set the serial port to specific settings, but that was all I've done on DOS.)

Can I write DOS code in D?

Sorry, no. D has a bit much overhead for the 16 bit world. C++ does, too, which is why to C++ for 16 bits has exception handling and RTTI removed.