Fertilization can occur when male semen enters the vagina at the right time of the woman's menstrual cycle. It is true that there may be a small number of sperm cells capable of fertilization in the pre-ejaculant (the chances of pre-ejaculate fertilization are much lower than in the case of an ejaculant), but for this to occur, the pre-ejaculant would have to be inside the woman's genital tract. Placing a T-shirt with a small amount of pre-ejaculant in the vaginal area does not create such a possibility.
The chances of fertilization in this case are therefore close to zero.