Really if it's only liverock, I'd probably leave it alone and let evaporation raise the s.g. till you can get some R.O. water again. 1.021 won't do any harm to the live rock, it will still continue to cycle (assuming that's where you're at?) And then when you can get some more water you can make a bit at say 1.025 or 1.026 (not all mind you ) make enough to replace what you take out of your tank and raise the s.g. that way. Aside from that just keep an eye on the other water chemistry specs in the meantime and watch for life on that rock; it's quite amusing.
SLC