Just how a Harlequin Masque works without an HQ selection is something that a lot of people are waiting to see when the codex is released. We have some information from someone with this weekend's White Dwarf that has some of the answers we are looking for, including that Light, Dark, and Twilight tables are warlord options.
This is from information provided by ShadowWolf 32387
This is from this weekends White Dwarf
A Harlequin army provides 4 choices for Warlord; Shadowseer, Death Jester, Solitaire, and Troupe Master.
Solitaires get no warlord traits
Death Jesters and Shadowseers get limited Warlord Traits
Troupe Master can choose from 40k BRB, the rules or from the Light, Dark, and Twilight Trait tables.