Today I wired up my garage remote to my high beams. It runs directly from the high beam switched supply, so no need for the 12V battery and it will never run out of juice. It's located under the seat in the battery compartment.
It basically involved soldering a jumper cable between the switch contacts so that the switch is effectively always closed. Then a lead is soldered between the negative on the remote's PCB and the negative on the battery, and another lead runs from the high beam switched supply on the bike and the positive on the remote.
This method will not work it you have a 3-6-9V remote. You'll need to use additional components. Also this method means that the remote broadcasts a signal every time the high beam is switched, whether from the high beam pass switch or the main high beam switch.
Whether this is a problem or not depends on whether your remote is the type that constantly transmits with the switch held or if like mine it sends one signal then stops until pressed again. Even if it's the former, it's not a particular problem in reality, since a) there are millions of combinations so you're unlikely to open up loads of random garages on your journey and b) it's just a radio transmitter, so the 'it'll wear it out quickly' thing is a myth, besides which they're like £6 on eBay.