Very few climbing harnesses have gear loops that are rated for full weight! Be very careful that you are using one of those if you are doing it this way! Most harnesses the only loop rated for human weight is the belay loop.
In-case it wasn't clear, using a gear loop on your "typical" climbing harness will result in failure if you load it with human load (eventually of not immediately).
I was planning on tying the lower block to the person belay loop along with a large biner. Then tie the working end of the line to that spare biner when I want to let go of it.
If my knot comes loose the downward slide should be very slow or not existent with both ratchet blocks engaged.