"What he needs is to not have his disguise seen through right away. If that's impossible, then we might just need a particularly clever lie."
[spoiler]Since Balors have True Seeing, it's going to be pretty hard to have any convincing disguise through magical means; any physical disguise is likely to be sufficiently convincing. Maybe since you're a cambion, you could be the half-fiendish offspring of some demon lord? A good, plausible lie might suffice, especially with a casting of glibness, and Marlowe's forgery might act as "proof" of your heritage. Thus, the lie becomes the "disguise," though honestly, not sure how much a Balor will care who the heck you are and how scary your daddy is, so to speak.[/spoiler]