Welcome to the forum!
I've been looking, but I haven't found anything concrete for you, yet. Some remotes have a sequence that will search through frequencies until you see your component turn on/off, but I don't see that mentioned in the manual for the 819 (or other Pioneer manuals that I've looked at this morning).
My VSX-23 manual does have a code listing for a Scientific Atlanta cable box, but I'm not sure it will help you. My remote uses five-digit codes, and I can't see a relationship between them and your remote codes for similar products. However, in case it might help, the code is 01877. That's the same code used for the VSX-919 and 1019.
The predecessor for your model, the 818, used three-digit codes. Here is the listing from that model for Scientific Atlanta set-top boxes: 705, 706, 708, 709, 731.
Sorry that I couldn't find anything concrete for you. You could always consider getting an aftermarket univeral remote control, too, if you really want to only use one remote.