I'm not sure what that even means. Recruit means make him her Knight, or at least that she would definitely offer him Knighthood. She might (and I say might) be able to 'offer' it to someone she knows cannot accept it, but she would have to present her 'offer' supercarefully to get around the 'no falsehood' compulsion, and Thomas (or whoever) could easily force her to tell the truth with even a simple direct question. It's an extreme stretch at best.
You can be recruited to go to a school, for a job, to join the military, but you don't have to take any of those jobs. Mab heavily recruited Harry for her Knight, he has close ties to the Fae, yet he could and did refuse the job. It was only when he was completely out of good options to save his daughter, and being Winter Knight was the least bad of several bad options did he then volunteer for the job. So yeah, she is telling the truth that she could recruit Thomas, but she also knows that doesn't mean that he would take it. I also think it has been established as least as far as Red Court Vamps go that the Fae might be able to put that part asleep [in Susan's case] but not cure the problem. So I doubt that Mab could get rid of the Hunger Demon. Now as we learned in Changes, part of the becoming the Knight thing is sex with Mab, perhaps that would allow Thomas to embrace Justine once more. However the kicker there that still doesn't make any sense, is if it was that simple, why didn't Justine or Thomas do it sooner.. Then is there a danger that Thomas could actually kill Mab during the mating ritual. Could Mab prevent Thomas from feeding on her during the ritual? What would that kind of food do to the Hunger Demon? Would Mab really want to risk that? One last note on offers and bargains, don't underestimate Mab when it comes to being super careful or super clever. Ask Nic in Skin Game, Mab didn't lie to him, and you could say she kept her word to him, but did it in such a way that he got screwed and she got what she wanted. That's the whole point of never make bargains with the Fae! You think because they cannot tell a lie, and they will make that a strong selling point in any deal negotiation with them, you will come out on top... You never do because there is always a catch, a glitch, something that you over looked so you lose, and the loss is on you because they never lie.
She can carefully phrase direct factual statements to imply a lie, or leave important information out, or otherwise lie with the truth, but the statements themselves must be factual at the time, or at least she has to truly believe they are factual at the time.
And so it was, she said she could
recruit, which was enough to freak Harry.. Why? Because Harry himself is so freaked out about becoming Knight after seeing what the mantel did to Slate.. Mab knows this, so she tells the truth, "I can recruit your brother!" Which Mab can, no lie, but Mab also knows that the mere suggestion of this to Harry sends all kinds of visions of horror dancing though his head if Thomas became her Knight. Mab also knows that Harry would do almost anything to prevent Thomas becoming her Knight, including becoming it himself, which what she wants.
There just isn't much wiggle room in Mab's recruitment statement.
What she is saying and what she is thinking are two different things.
Yes, she wants Harry. I'm sure she sees Thomas second best on several levels. But if Harry is dead (really dead, I mean, gone and not coming back unless God wills it), then Thomas would be her next choice, because then she can't have Harry anyway. Or at least, Thomas would have been her choice at that time, whether that's still true years later is another matter.
Now you are getting into hypotheticals, so Mab can say all of the above and be telling the truth in a "what if" world.. However whether it would actually come about is another matter for a number of reasons.. So Mab in that case can tell the truth without being committed to doing anything, which she would and does gleefully tell those who thought they had a deal with her, but didn't get what they wanted.