Ok, bit of info – There's no issue with regards U-Turn adjustment on the fly as Trailmonkey said, it is designed to allow this function. Although the adjustment can be made easier by removing 1 of the ball bearings from under the U-Turn cap. You should clean under the U-Turn adjuster anyway to keep it free and clean from dirt, most problems usually arise from crap under there or the lube being washed out by jetwash / degreaser.
The compression and damping circuits are both in the top part of the fork, it's only the adjustment of the rebound which is at the bottom of the fork. The compression and rebound are isolated from the spring rate, and as such have no influence. If you are having to adjust compression with U-Turn wound down then you may have other issues.
The lockout function or Flood Gate is basically a pressure relief valve that restricts oil flow under noraml use but allows oil flow through the compression circuit under high fork compression, if the Flood Gate is closed too much then the pressure required to open the valve sometimes overcomes the O Ring resulting in an oil leak. Operating ther U-Turn will do little damage to this as it just increases the pre compression pressure required to over come the Flood Gate. Only if the Flood Gate threshold is too high damage may occur.
In short then, yes it's ok to use U-Turn on the fly!