Inspired by wetmartini, I decided to try an officetel service I found on a local site called Bamdae (
https://www.bamdae.com/op). You'll need to be able to read some basic Korean to navigate this scene but with a little legwork and some Google Translate, it's possible to book via SMS.
I chose a service called "GG" (easy to find if you just search the posts in the link above) and sent a message to the mobile number in their ad. It's probably better to do this from a local Korean mobile. The girl I wanted to book was off so I went for an hour session with "Mint" (민트) for KRW 140K. There was some back and forth about my nickname on Bamdae but eventually after 5 text messages and one awkward call, I had a final confirmation and an SMS with the address near Gangnam station. Took the bus down there from my office and found the building pretty easily. It's just a 2 minute walk from exit #1 in a large Daewoo officetel. I had instructions to go to a particular apartment but when I got there a young guy showed up and asked me for the money up front. At this point I didn't have much choice so I gave it to him and he told me to go to a unit on another floor, which I did. Rang the bell and walked into a decent officetel unit fully fitted out to look like a normal apartment. Mint was quite young and attractive, probably no older than 22. She was a little awkward to be honest and we made some chit chat in Korean before taking a shower.
Long story short: this was my first officetel experience and maybe I got lucky but the girl was young, super tight, quite pretty, and not jaded. I will definitely be back. For the price, I haven't come across anything like this in Seoul, and having been based in Korea for a while and seen some pretty crazy things, that's saying something.
Happy to help if anyone has questions though I don't check PMs very often.