There are a bunch of factors to consider here. To name a few:
- As you mention, zero to very little gravity will tend to increase the utilization rate as there will be less competitive inhibition from wort sugars.
- Boiling in water alone will mean a higher pH (as malt phosphates, even in extract brewing, would normally react with hardness in/added to the water to naturally lower wort pH). Of course the exact pH of the boil will depend on your water supply, but high pH favors the dissolution of iso-alpha acids and will also increase utilization rates.
- The Mg++ ion catylizes the isomerization reaction and should be more abundant in water than it is in wort (where it would react with phosphates [see above]). Higher levels, therefore, will favor more isomerization, and again increase utilization.
- You will lose out on good hot-break formation, which is largely malt-derived protein agglomerating with malt/hop-derived polyphenols and precipitating as trub. Trub is known to carry iso-alpha acids out of the wort with it (hence its bitter taste), so once more your utilization will go up. You might also experience haze problems in your beer because of the unprecipitated malt protein (which only matters if you care about hazy beer).
- Iso-alpha acids stick to yeast cells in fermentation, so if you're giving your yeast wort with very high leves (and this is not specific to this case, but a general occurance) it may not preform as well.
My guess is that calculating IBUs is going to be near-impossible outside of either relying on sensory evaluation of the finished beer or sending a sample to a lab for verification. One thing you can obviously say, though, is that the utilization will be significantly higher.
Also, remember that along with higher levels of iso-alpha acids you should see higher levels of other hop resin byproducts, not all of which are pleasant to taste. I've often read reports that over-utilization of hops in the kettle leads to a less agreeable hop flavor in the finished beer.
That being said, the risks might not have such negative impacts on your finished beer, and at the very least you could probably correct your utilization numbers by trying this procedure a few times and adjusting accordingly by taste. If it ends up being something that's convenient for you, why not go for it?