Just about to rack my Belgian Ale, but I know the IBU is too low. At this point, instead of dry-hopping, can I boil a cup of water with a OZ of hops and dump it in at second racking?
That is not really going to isomerize the hops unless you boil for a long time. A few drops of hop extract would be helpful here: https://www.morebeer.com/products/isohop-bitterness-extract-1-oz.html Cheers!
I say yes you can. Go for it. I'd boil for at least 15 minutes, maybe longer, depending how many IBUs you want to add (longer boil will give you more bitterness). If you're mathematically savvy, you can even calculate how much volume you need and how long to boil. There's a practical limit of about 90-100 IBUs that can be dissolved, but if you add a quart of 100 IBU wort or water vs. just a cup, you can figure out how much this adds for the whole batch by simple division. For example, 1 quart of 100 IBUs added to 5 gallons will give you 1/20 of 100 = 5 IBUs. So maybe you even want to add a whole gallon of wort, depending how far short you are.
I would just let it ride. Sometimes the beers I thought were pretty awful during brewing and fermentation came through carbonation and conditioning as a great beer.
How much is too low? I tend to lean with SFACRKnight and have had similar experiences. On the other side, if you are determined to add more IBUs I think that the hop extract is the better way to go, it will be tough to hit exactly what you want with the boiling technique and it may not blend as well as you would hope.
Dryhopping may add perceived bitterness, but without isomerization the hop flavor and aroma will be decidedly different.
