Will frozen pearl onions caramelize as easily as fresh ones will?
I don't have any experience with frozen or canned pearl onions (I assume canned because of the mention of draining), but I regularly cut and freeze 10lbs of onions at a time for ease of use later in the week/month (more info on how/why here).  

With these onions that I freeze myself, caramelization seems to be the same as it would be with fresh onions.  I haven't done anything special when caramelizing with them, and there's been no increase in necessary cooking time or decrease in flavor.  

If you really wanted to be safe, you could probably drop them in boiling water for a minute or two (or defrost them on the counter for an hour), but with how long it's going to take to caramelize anyway, I would think this extra step is unnecessary.

Also: if you're going to take the time to caramelize, but you're short on time during the week after work/school/etc, I recommend making a huge batch and then freezing the resulting caramelized onions as well.  Very handy!