A lot of what works in the Garmin folder on older units will also work in a Garmin folder on an SD card. The same goes for later models, where what's in the .System folder as ancillary files will also work in a Garmin folder on a card. The *.srd files do need to stay in the unit's ASR folder though as you've pointed out.
That's dead right about the 8xx series ASR files being hard to get. Like hen's teeth now, but just in case you missed them they are available in the CN EU 2014.20 map thread here: [Only registered and activated users can see links. ].
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