Xantech's CC12 (one DPDT relay on a chassis) or CC62 (six DPDT relays on a chassis, if they still exist, are IR controllable DPDT relays that will let you do all sorts of things like what you did.

Years ago I worked out that an amp's headphone output is generally a power amp channel's output fed through a 300 ohm or so resistor (one per channel, natch) to keep from blowing up headphones, so you could take the power amp out and route it directly to speakers, then switch it to headphones through the resistor. No need to open the receiver.

In fact, opening the receiver means that any time that component needs service it needs some custom care. The above-suggested relays let you do what you need without having to modify the receiver in any way.

Yes, cable boxes always have something in them like five volts... to make the IR sensor work.