A potent attacking partnership for the Roar over the last two seasons, who may again team up next season in Germany, the pair created two first-half goals in the AFC Champions League dead rubber on Wednesday night at Suncorp Stadium.
Maclaren scored in the 18th and 37th minutes with the lively Borrello providing the final pass on each occasion.
Both players are bound for Europe next season and could even end up at the same club, with Borrello moving to FC Kaiserslautern and Socceroos hopeful Maclaren also poised to join the German second division outfit.
The goals put Brisbane 2-1 up after an early blunder by goalkeeper Jamie Young, who conceded a ninth-minute own goal when he fumbled an attempt from Seo Myeong-won at the back post.
But the Roar conceded twice more to let the match slip in front of just 3,245 fans - the club's worst AFC Champions League crowd at home.
Maclaren had a golden chance to seal his hat-trick from close range in the 53rd minute but somehow hit the woodwork with the goal at his mercy.
The ball surged up the other end and seconds later, Ulsan made it 2-2 as Nam Hee-Cheol capitalised on a horrendous backpass from Tommy Oar, who was playing out of position at left-back.
Then in the 76th minute, substitute Kim Young-Jin caught Brisbane's defenders napping and poked the ball home from a free kick to put them ahead.
The Roar had enough opportunities to win the match, with Brett Holman hitting the post in the first half and Nick D'Agostino also going close with a header in the second half.
Maclaren's brace took his final career tally at Brisbane to 43 goals in 60 appearances in all competitions, strengthening his case for a call-up for Australia's must-win World Cup qualifier against Saudi Arabia next month.
Thomas Broich did not feature due to an ankle issue but the two-time Johnny Warren medallist is also leaving the Roar after seven seasons and told Fox Sports before the match he was content with his decision.
"I feel like my work here is done. It's time for a new chapter," Broich said.
However, the 36-year-old said he was in "no man's land" and has received no offers for next season, conceding he may end up retiring instead of playing on.