Theoretically, if you sit reserve and do part 91 non-revenue repositioning legs (which I do several times a week) you could log PIC if the captain was willing to let you do it (he would have to log SIC, which is legal for him).
We have little to no paperwork for a 91 flight, so there's not much official papertrail, but obviously the company ASSUMES the captain is going to be PIC...
The two big downsides to doing this are:
1) The company would be really ****ed if they found out.
2) It would be a little hard to explain to a prospective employer...they would understand you were legal, but they would also understand that your company would not have approved.