I'd like to switch away from my current carrier to visible, but for various reasons I don't want to port my current number right away. Is it possible to go through the signup process without porting, getting myself an essentially temporary new number, then a month or so later port my old number over and essentially discard my new/temporary number?
Yes, according to what I read in the port in procedure you will get a temporary number and you need to port in within 30 days. Do not cancel the other carrier or you might lose your old number.
If you have an iPhone you could try the 15 day free trial.