The most thorough study of messianic interpretation in the targums is S.H. Levey, The Messiah: An Aramaic Interpretation (New York: Ktav, 1974) which conveniently provides a list of texts that give a messianic slant to (often) non-messianic texts in the Hebrew Bible. The list is cited on the hope of Israel missionary website (I'm aware of Levey's work but I confess that I haven't confirmed these citations just yet).
Gen 3.15 (Pseudo-Jonathan): "They are destined to make peace at the end, in the days of King Messiah"
Gen 3.15 (Frg.): "They will make peace with one another in the end, in the very end of days, in the days of King Messiah"
Gen 35.21 (Ps.-J): "And Jacob moved on, and pitched his tent onward to the tower of Eder, the place whence the King Messiah is destined to reveal himself at the end of days"
Gen 49.1 (Ps.-J): "As soon as the date of the End when the King Messiah would arrive was revealed to him..."
Gen 49.1 (Frg.): "For he was revealing to them all that was going to occur at the very end, the time of the Messiah."
Gen 49.10-12 (Onq.): "The transmission of dominion shall not cease from the house of Judah, nor the scribe from his children's children, forever, until the Messiah comes, to whom the Kingdom belongs, and whom nations will obey."
Gen 49.10-12 (Ps.-J): "Kings and rulers shall not cease from the house of Judah, nor scribes who teach the Torah from his seed, until the time when the King Messiah shall come, the youngest of his sons, and because of him nations shall melt away....How beautiful is the King Messiah who is destined to arise from the house of Judah...How beautiful are the eyes of King Messiah, as pure wine!"
Gen 49.10-12 (Frg.): "Kings shall not cease from the house of Judah, nor scribes who teach the Torah from his children's children, until the time of the coming of King Messiah, to whom belongs the Kingdom, and to whom all dominions of the earth shall become subservient...How beautiful is he, King Messiah, who is destined to arise from the house of Judah....How beautiful to behold are they, the eyes of the King Messiah..."
Exod 12.42 (Frg.): "Moses shall go forth from the wilderness and the King Messiah from Rome..."
Exod 17.16 (Ps.-J): "from the generation of this world, and from the generation of the Messiah, and from the generation of the World-to-Come."
Exod 40.9-11 (Ps.-J): "...and consecrate it for the crown of the kingdom of the house of Judah and King Messiah, who is destined to redeem Israel at the end of days...and from whom is to descend the Messiah son of Ephraim, by whose hand the house of Israel is to vanquish Gog and his confederates at the end of days."
Num 11.26 (Frg.): "At the end, the very end of days, Gog and Magog and their armies shall go up against Jerusalem, but they shall fall by the hand of the King Messiah."
Num 23.21 (Ps.-J): "The Memra (word) of the Lord their God is their help, and the trumpet-call of the King Messiah echoes in their midst."
Num 24.7 (Frg.): "Their king shall arise from among them, and their deliverer shall be of them and with them...Exalted shall be the kingdom of the King Messiah."
Num 24.17-24 (Onq.): "I see him, but not now; I behold him, but he is not hear; when a king shall arise out of Jacob and be anointed the Messiah out of Israel."
Num 24.17-24 (Ps.-J): "...but when a mighty king of the house of Jacob shall reign, and shall be anointed Messiah, wielding the mighty scepter of Israel....to wage war against Israel, in the days of the King Messiah...and those shall fall by the hand of the King Messiah..."
Deut 25.19 (Ps,-J.): "Even unto the days of King Messiah, you shall not forget."
Deut 30.4-9 (Ps.-J.): "and from there he shall bring you near by the hand of the King Messiah..."
I Sam 2.7-10: "He shall give strength to His king, and shall make great the kingdom of His Messiah"
I Sam 2.35: "I will raise up before Me a trustworthy priest, who shall minister according to My word and My will, and I will establish for him an enduring reign and he shall serve my Messiah all the days."
2 Sam 22.28-32: "and the deliverance which Thou shalt perform for Thy Messiah and for the remnant of Thy people..."
2 Sam 23.1-5: "...Said David, the son of Jesse, said the man who was anointed to the Messianic Kingship by the Memra of the God of Jacob...God spoke to me...and He decided to appoint for me a king, he is the Messiah, who is destined to arise and rule in the fear of the Lord..."
1 Kgs 5.13: "who were destined to rule in this world and in the world of the Messiah"
Isa 4.1-6: "At that time the Messiah of the Lord shall be a joy
Now obviously the vast majority of these glosses belong to third or fourth century textual witnesses and developed traditions about a messiah from the talmudic period. However, if we can use 12th century AD masoretic texts to reconstruct 8th century BC Israel, surely we can lean on 4th century texts/traditions to shed light on 1st century (even pre-70 AD) Palestineian Judaism (esp. since a similar messianic embellishment can be found in Old Greek texts). Generally speaking, I think where you have a Targum reading paralleled by a NT, DSS, LXX, or Philonic reading, then you have a reasonable case that the specific targumic reading is probably fairly early.