By what I've gathered, information wise, the only way $BTC would be used is if some catastrophic event happened to somehow depeg $UST.. otherwise the normal protocol would still burn/mint $LUNA to maintain peg. Could be wrong about this but just what I think!
From my understanding the reserve LFG is building will be a way for ppl to cash out their $UST for something other than Luna, nothing to do with the burn mechanism