You certainly can do it with some cautions. I was wondering the same thing at one point and sent an email to the company asking if it was ok. Here's what they responded with.
Thank you for submitting your question about our product B*L*CŪ.
I would not recommend a prolonged storage situation with B*L*CŪ in a keg for the following reasons.
1. B*L*CŪ is an alkaline liquid. Even in use concentrations the pH of the solution is going to be highly alkaline ... depending on how much you add to the water in the keg. If you are using a standard beer keg the metal of the keg is going to be in constant contact with the alkaline solution. Over time ... over repeated storage cycles ... you will start to cause corrosion to the metal surface. Now, you won't eat a hole in the keg overnight, but with time that surface will get pitted and any future use of that keg for other functions might get limited. That pitted surface will be harder to clean at some future date if you were to put beer in it.
2. If you use CO2 to push the cleaner thru your system you are potentially going to reduce the cleaning power IF you keep constant CO2 pressure on the keg. Chemically CO2 will go into the water and form carbonic acid H2CO3. This mild acid gives the sharp taste to beer that has been over carbonated in a keg due to too high a CO2 pressure. Constantly pushing CO2 into a keg containing an alkaline cleaner has an accelerated action of the mild carbonic acid being immediately neutralized by the alkaline OH in the cleaner. This chemically pulls more and more CO2 into the solution. Over time the pH of the alkaline cleaner will start to drop due to the mild acid neutralization. Now, the initial alkalinity of the cleaner is so high that this is not a factor in cleaning, BUT if this storage under CO2 goes on for weeks or months? the loss of alkalinity might result in reduced cleaning action.
3. Your question did not mention the conditions under which you are storing this keg. There is always the overriding concern of safety. A keg of liquid that is highly alkaline is potentially dangerous if someone gets into it. It might get tapped as a keg of beer. A child might have access to the keg and release the cleaner causing chemical burns to skin and eyes ... especially if the keg is under pressure.
Summary ... our chemical cleaners for beverage systems are in use every day by both professional technicians cleaning commercial systems and private individuals cleaning home equipment. Most make up a cleaning solution that will be used up within a matter of minutes or maybe a few hours. The cost of a cleaning application is so economical that there is not much financial motivation to either save the solution or store it for future use. The convenience of mixing a liquid cleaner into water is such a quick process that the time saving of mixing a larger quantity for future use is fairly inconsequential.
I would be happy to pursue this further with you if you have any question or comments.
A couple of the cautions are no brainers, but some are things I wouldn't think of such as using CO2 to push it will neutralize the alkaline solution. This can be remedied by bleeding the keg after every use or using plain old air to push.