I think BTCGuild was accused of this recently.
The bottom line is, yes, it is possible, and no, you couldn't detect it if it was well-hidden.
If you use a PPS derivative reward scheme it doesn't matter.
Since the operator has only two options, to pay for your submitted shares or get labeled as a scammer
PPS fraud would be instantly visible within less than a week, since there is zero variance & multiple miners would be reporting 2-3% less than average earnings suddenly.
Maybe if you take 0.1% or a similar amount, it would be brushed off as a normal occurrence.
Doesn't sound too profitable though.
Bigger pools can already earn enough without risking their entire reputation.