I believe this will accomplish what you want, but with a different workflow. You need to think in reverse...
1. Adjust the loop start and end,
not the clip start and end, to define the parts of the selection you want. SONAR will keep looping while you do this.
2. When the loop length is just right, select the track (if it isn't already) by clicking on the track number header.
3. Float the cursor just above the timeline so it becomes a magnifying glass, then click inside the loop brackets. This defines the part of the clip that's looped as the selection.
4. Ctrl+click on the clip handle anywhere within the loop bracket, and then drag the selection where you want. If you don't have ctrl selected, then you can extract the selection from the clip.
The only difference compared to editing the clip start and end is the loop brackets can't snap to audio zero crossings, only whatever snap you specify. Of course you can turn off snap (keyboard shortcut N) and place the loop start or end manually on a zero crossing.
Check out this technique and see what you think.