It's possible to set arbitrary start times for selected tracks atm if you use Cue files.
Select a track, you want, make a cue sheet for just that track and include in it how far into the track you want it to start. Then import that cue file into MC and tag it as if it were the oringinal file.
This will work for long radio mixes when you dont want to hear the grp being introduced and any bla bla that precedes the act. Note that the cue method is good for defning start times only, there is no way to define an end time.
There is one problem tho, you would have to repeat the above tedious process for each track. At this point i hear a huge collective *GROAN*
Maybe create a cue sheet that will act as a template, with the times you want, make multiples copies of it for each track and edit the FILE item to point to the proper file.
Still tiring, but its the only way you can get what you want now, wihout using the preview option.