A workable solution has been achieved:

On a 3-to-1 AV switch:
NES
SNES
XBOX

On a 2-to-1 coax switch:
NES top-loader
ColecoVision

On the way: a HyperSpin box that will use HDMI. Pics later this week.

I found the AV adapter for the SNES in box of extra cables. The XBOX was in another room, but moved to this setup when an AV opening became available.

Many thanks to everyone for their insight and expertise!

-jeff