Not a huge Deadmau5 fan, but I like some progressive techno, and it's ok to like different music than other people
So. Given that I haven't heard the examples you give, here are some ideas:
-Yes. Multitrack. Try recording a note with an automated filter sweep to midi, capturing both the note and the automation. Play that midi, recording the audio. Then copy the midi to a new track, keeping the automation the same but changing the note. Playback, record to new audio track, repeat.
-Poly it up! I can't afford multiple voyagers, but given that you can get three used slim phatties for less than the price of a used voyager keyboard (or four depending on ebay season), I have three phatties. I modulate them all simultaneously with external cv stuff like the cp-251. And they sound absolutely f--ing gorgeous doing huge house-y chords
-arpeggiate a chord, either with a seq or by hand (like the cavemen once did!), while you run it through a tempo-synced delay.or a huge reverb. This won't sound like a single chord, but the delays or reverb tail will have all the chord's notes ringing through simultaneously.
-change the frequencies of osc 2 and 3 to make a chord. Many presets have this going on. If you are super slick with your midi skillz, you can use midi automation to change the frequencies (thus changing the chord) over time. Use the midi implementation chart at the back of the voyager manual to see which cc's control what. If you have Max for Live, there is probably a patch some genius wrote that might help you do this.
-cheat. Use plugins, that's most likely what yr boy Joel is often doing, despite his massive room full of toys. I'm sure he uses his modulars on stuff, but there are some pretty rad plugins that he's probably using when he writes on tour, etc. I use NI's Monark along with some clever max for live routings to get three Monarks going poly on three different tracks at once. As they all have randomness and voltage leaks programmed into them, the three instances wiggling together sounds pretty damn close to my tri-phatty.