Yes, what you said about remote learn is what I always do. I "learn" the codes through the computer with the remote connected with the serial cable. I have "learned" hundreds of codes that way and only had to edit a handful. I will admit that sometimes you have to repeat the learn for a particular code two or three times, but not often.

Since this message is from 5 years ago, I figure you have perfected your method as well and it's a moot point, but I just wanted to say that I have been successful also.

I'm really here to update my OP regarding the use of a USB to Serial adapter that I mentioned in the OP. I have an ATEN brand adapter and just built a new computer that does not have a motherboard serial port on it. So I was forced to make this ATEN adapter work.

I connected the ATEN brand USB to Serial Adapter to the front USB port of the computer and updated the driver from the latest file on the ATEN website;
[Link: aten.com]
Then in Device Manager I set the serial port to
19200 Baud 8-N-1 Xon/Xoff - FIFO buffers set themselves to ATEN's defaults, I set the port to COM1 in Device Manager also under Advanced settings.
Using either the original Serial cable (4 bands or rings on the 1/8" plug end) or the one on the ATEN adapter (3 bands or rings), I could communicate with the Pronto 1000 without any problem.