Tim,
What you are describing is implemented on A cats regularly,
Ronstan makes a block that works well for this puropse.
Part #
RF 62175 low lead ratchet with cam

Alternatively you can buy it witout a cam.

These blocks have awesome holding power much better then the harken ones, but they do chew up line quickly so you will need to replace your mainsheet more often

Cheers
Eric Anderson