Can fiber caplets be used as a substitute for powdered psyllium husk?

It's not a good idea. Whilst you may get away with it, it can be tricky to get the dosage correct. Remember, the caplets are designed for people and therefore much more concentrated fiber. In addition, there's a risk of choking if the dog doesn't drink water straight after taking the caplet. Also, always read the packaging and remember that any product containing an artificial sweetener (such as Xylitol) is NOT suitable for dogs. If your can't source psyllium husk, then consider other options such as pumpkin or the fresh vegetables suggested above.