It is IMPOSSIBLE for the thief on the cross to be baptized. IMPOSSIBLE.
Christian baptism was instituted by Jesus after His resurrection (during the Great Commission) and before his ascension. The first Christian baptisms occurred on the Day of Pentecost (Acts 2)...some 53 days AFTER the thief's death.
Saying the thief was not baptized is like saying Erza or Nehemiah or King David or Issac was not baptized.
The thief was saved exactly like all the other saints in the OT listed in Hebrews 11. BY FAITH.
I don't think you are taking everything into account. First of all, the theif was literally dying with Jesus. Why would he need to be baptised only to show his intention of doing so, when he is ACTUALLY doing it?
Second of all, if we die with Christ, and rise with him, we can only mean this in two ways. Either, it is only a foreshadowing of what is to come - the resurrection. So if we die with Christ, we will resurrect with Him on that day. Or, it is an actual dying as soon as you go under the water, and as soon as you rise up, you are alive with Christ from that moment.
But the other interpretation has some problems; it is not stated, that as soon as you rise up from baptism, you are alive with Christ. Just like Christ said, on the day of judgement he will say to the goats, he
never knew them.
Thus, if we take the parable of the virgins in to the account, we learn that the process of getting to know Christ has a threshold, and that threshold is not met or predicated on baptism, but the individuals preseverence of the Word - in a quantity that meets the requirements of the Wedding.
Did not the disciples get baptised? Did the people who got baptised by John the Baptist - Jesus himself being one of them - need to get baptised again?
Regarding the salvation of the thief. I think it is worth ponding the initial premise of the Bible. That the wages of sin is death. Once a person has died for his sin, the price for his sin is also paid. However, as Jesus takes the punishment for sin, there is the possibility to live again - and according to Jesus, not even die. The thief then, due to his subordination to Christ, and his comittment unto death, will be resurrected with Christ. Thus foreshadowing the resurrection is not necessary for him, because he is in the process of doing it.