I used the both the standard metering pump as well as the switched circuit adapter and a high volume peristatic to feed a kalkreactor, not the deltec but they are all basically the same animal.
Here are the issues you'll encounter.
1) The volume pumped will be reduced somewhat. This means your pump will likely need to cycle longer to pump the same volume of make-up water. If you are currently near the cutoff threshold the additional time required might make your osmolator go into the cutoff mode (alarm). Highly unlikely unless you are pumping back a massive amount of water or pushing through a very long section of tubing.
2) The sump chamber size your osmolator sensor in is going to determine how much make up water is pumped back. If you have a giant unbaffled sump, the osmolator is going to need to pump back a huge amount of water each time, this may cause a pH spike. However, if you are using a traditional sump with a small return chamber (this is ideal in this setup), you'll be injecting only small amounts of kalk each time, so no spike, no problems with cutoff. This is a rare problem and would only really happen if you were using something like a 150g Rubbermaid as a sump.
The tunze works great in this scenario and you should have no issues at all. While I don't particularly care for the supplied metering pump, the switched socket adapter works very nicely controlling a high volume peristaltic pump (I'm using an adjustable mecomatic pulsafeeder) that lets me control the flow rate with incredible precision.
You might want to do a bit more research on the operation and design of kalk reactors before you commit to buying some very expensive equipment.
jb