I think that it works out to be about 20% N (it can dissolve at higher concentrations, but it requires work). So more water than putting on 28 0 0. Might just be able to water down your 28 and have the same results.
UAN will also not gas off as badly as urea, so that has to be considered.
Here is a segment from U of Mn.
Uniformity of particle size is important with dry solid urea, whether applied directly or in blended formulations. Some imported urea appears to be below U.S. quality standards on granule uniformity. Dissolving urea and marketing the liquid solution is an attempt to overcome this lack of uniformity and still take advantage of the favorable urea price.
The liquid mix of urea and ammonium nitrate (UAN 28% N) has been on the market for a long time. The characteristics of this solution, however, are not the same as when urea alone is dissolved in water. A solution of 50% urea by weight results in 23-0-0 and has a salting-out temperature of 60 degrees F. In order to store and handle liquid urea during cooler temperatures, the nitrogen concentration must be lowered to reduce salting problems. There are several possible formulations that can be used for this, such as adding small amounts of ammonium nitrate, ammonium sulfate, or anhydrous ammonia.
Research, particularly on liquid urea, is very limited. Generally, where dry urea functions successfully, the fluid urea should perform equally well and may have the advantage of better uniformity over some dry urea sources.
Make sure you figure out all your costs if you want to try it. I imagine that UAN will be very similar in cost by the time you get everything done. Unless UAN is not available in your area.