So I built a small IR transmitter (using an IR LED salvaged from an old remote - so can't say what model IR LED it is) and hooked it to the GPIO pins of my Rpi 4.
Using info from several different web sites - I was able trigger the Nikon D90 shutter using this cobbled together IR remote.
In addition - I can confirm, that if you setup the D90 so the shutter is triggered by the IR remote and shutterspeed is set to "- -" (it doesn't say "bulb" in IR remote mode) you can open the shutter with the 1st press of the IR remote and close it using a second IR remote signal.  I was able to take a 67sec exposure using this trick.  Next I'll see if I can get the GPIO pin controlling the IR to trigger using the GPIO Serial input on the RPi.  If I it get it all working - I'll post the whole setup here. But that's gonna have to wait.  Cost so far - nothing but time and and old unused remote control.