It actually happens concurrently with Barbara Hambly's Children of the Jedi novel, during which Mara appears with Lando on the other side of a holocom call.
“What? Who is it?”
Leia prodded her husband’s shoulder. “I told you you should have waited for her to call back.” She turned back to the holo image of the woman in the field, fiery hair tousled, green eyes blinking into the dim glow of the lights on her end of the transmission. She wore a gold chain around her neck and a shirt Leia recognized as belonging to Lando Calrissian. “Mara, I’m sorry …”
“No, it’s all right.” Mara Jade rubbed her eyes with a quick gesture, and that seemed to take care of any residual sleepiness, as if she’d clicked off a switch. “I must look like one of the Nightsisters of Dathomir. What time is it where you are? What’s up? Is there a problem?”
Mara made an extremely unladylike comment and Lando Calrissian appeared behind her shoulder, waxed and combed and dressed in his best purple satin for an evening out.
-- Barbara Hambly: Children of the Jedi, Chapter 12
No mention is made of the fact that they are undercover, which, as you said, is in fact something Timothy Zahn put into his Vision of the Future. I doubt that Barbara Hambly had intended it as an undercover sting when she wrote it, more likely she did it so she could have Luke freed up to pair him with her original character of Callista (but that is just my assumption).