People used to do this all the time in the old top 50 (which existed before the arena was created), so it's hardly unprecedented. Some people still do this in their arena decks; they just don't last very long, anymore, since a few losses and they drop below the top 500.
If you feel guilty about giving away 'free' rares, you could fill the deck with rares and shards, but also use some quantum pillars or something so that the player still has to do a little bit of work to get them. Depending on how many shards/rares you own, you could even make decks like 6x pharaoh 6x shard of readiness, or 6x miracle 6x morning glory 6x flying weapon, or whatever, that are still a proper deck, yet give a good chance at rares if they are beaten.