Lithuania international Zaliukas, who made over 200 appearances for the Jambos, is currently training with the first-team squad, while McDermott spoke of his frustration at failing to land any of his six other loan targets.
"He came on the radar a long time ago," McDermott told the club's official website.
"He's been at QPR and he's an experienced player. He's here for a number of days and he has a good pedigree.
"He's 29, he's captained Hearts and to have that bit more experience is important.
"We have some good young centre-halves here, but to add to that with some experience would be good."
McDermott added: "I've made six enquiries for players we targeted.
"For whatever reason we couldn't get them over the line.
"I know I've said it before, but I'm not going to take players for the sake of it.
"There is money to bring loan signings in if we can find the right one."