I have a cyser that is just too dry and I was planning to try using tannin to even out the flavors. I was going to bottle soon, and was going to use campden tablets and sweeten with apple juice concentrate with sorbate.
Would there be any problem with using tannin at the same time as the other ingredients right before I bottle it? Tannin needs time to change the flavors right? Would the campden tablet and sorbate interfere with this process?
There shouldn't be an problem with using tannin along with the other ingredients. On the other hand, are you sure you want to use all of that? A dry cyser, assuming you started at a good starting gravity, should have enough alcohol to kill off the yeasties. Exceptions would be alcohol tolerant yeasts from the bayanas strain. Be sure to let the cyser sit for a good long time before adding the concentrate. The cyser must be clear -- clear enough to read newspaper through the carboy. Cloudiness could be suspended yeast along with the colloidal material that might activate with added sugar.
I've done this many times, added a little honey to sweeten a dry mead without the yeasts re-activating, but this method needs to be done carefully.
Another possibility is simply to let the cyser age. I did this with a dry cyser, very rough tasting, with a finishing gravity of about .998. Amazingly, an aging period of about 3 years or so did wonders for it. Taste improved, body and mouthfeel were good.
Hope this is helpful!