Scull cap sounds good.
I've never seen helmet liners sold. Always made my own by blocking the vents with clear vinyl or clear packing tape and then adding a swatch of fleece (using double sided tape).
EDITED: My Liner goes on the inside of the helmet
If a balaclava won't work, how about a narrow scarf instead and maybe some of that anti-fog stuff mentioned in the Commuter's forum (if he doesn't already have it).