Laptop batteries wouldn't be any good as they don't usually have 12V.

What I'd recommend is 2 RC car battery packs used in serial to give 14.4V. If you used 3000mAh packs then you would expect 3+hours. I'm not sure if we've ever come to a consensus about curent requirements, but we know that it's under 1A including disks being spun up and the VFD on. With a judicious use of the hijack screen blanker and mp3s with a reasonable bit rate double to triple that 3 hour time should be possible.

You could buy or even make 10 pack cells (=12V) but then you'd need a charger capable of charging them, which would put you into RC airplane territory. I'm also not sure at what voltage the empeg considers to be undervoltage. If it's exactly 12V then you wouldn't be able to maximise the usage of a 12V pack's capacity.
Sticking to 2 six cell packs gives you a wide range of RC chargers (including RatShack's cheapies) and avoids that problem.

The other heavier and bulkier options are either a 12V motorcycle battery (lead acid - ugh) or 9 D cells. IIRC, a Duracell D cell is approximately 8000mAh, so that would give you nearly 3 times the duration of the RC packs, although that would come at the expensive of weight. bulk and the fact that you couldn't recharge them.