Your best bet would be something that runs on Haswell. Unfortunately there are many different models and ultrabooks are likely to use the low-tier ones for their lower power consumption (and lower performance).
HD Graphics (GT1, 6 execution units)
HD Graphics 4200, 4400, 4600, P4600, P4700 (GT2, 20 execution units)
HD Graphics 5000 (GT3, 40 execution units, twice the power-performance of HD4xxx for compute-limited workloads, 15W TDP SKUs)
Iris Graphics 5100 (Same as HD Graphics 5000, 28W TDP SKUs)
Iris Pro Graphics 5200 (GT3e, Same as previous, but with addition of large embedded DRAM cache to improve performance of bandwidth-limited workloads)
Second best bet would be to wait until last quarter of the year and see what AMD's Kaveri APU will bring. I bet it has far better performance as far as GPU is concerned, especially if it runs on DDR4 RAM instead of DDR3. The question is whether Kaveri APUs have low enough power consumption to find themselves in something equivalent to ultrabooks. Ultrabook is an Intel thing, so you won't find AMD hardware in them.
You said ultrabook is a compromise, but you do realize that ultrabooks are like the thinnest and lowest powered laptops around? If you really want a compromise, you'd look for something "heavier" than ultrabook. A real workhorse laptop that can actually have some computing power. Such a thing would have either NVIDIA or AMD mobile GPU and far more likely to run FSO now and in the future.
If possible, I'd wait until mobile AMD Kaveri APUs with DDR4 support come available. They should be real nice workhorse laptops with some adequate computing power.
Edit: Before you ask about DDR4, it's because one of the biggest limitations integrated graphics face is memory bandwidth. Discrete GPU's have on-PCB GDDR5 or GDDR3 on the low-end cards, while integrated are limited to DDR3. DDR4 will bring bandwidth much closer to GDDR3/5. DDR4 does have many other improvements as well other than raw bandwidth, and AMD's own hUMA architecture brings even more efficiency to the table. Couple AMD's Kaveri with DDR4 and you have lots of potential. The question is, can AMD deliver.