Not practical at all. Most probably the users would be just pissed of. Right now it is not even practical to mine with GPU-s, not to mention CPU-s and for sure JS.
Regarding the question:
"if I put the user in as part of a mining pool, ..."
No, it is not important that he would find a hash. It is working this way. Your all workers are calculating hashes (does not matter if you find the right one or no). If a miner mined something at that time, the miner calculates how much work you have done and gives you the amount of bitcoins. So if you contributed 5% - you will get 5% of 25BTC and in case of some pulls fees. If pull mined nothing, you get nothing.
So in total, if you do not want to piss of users, do not do this.
1You will have to explain to the revenue service why you make this income in bitcoins. Using the website visitors' electrical energy for mining bitcoins could be seen as theft. – user3284699 – 2016-01-27T19:38:30.283
This was a question well ahead of its time. See COINHIVE and MONERO! – PGSystemTester – 2018-07-24T19:41:42.377
@PGCodeRider Thanks! I was just in high school – Jack Humphries – 2018-07-25T16:36:59.107