HKBanker

Generally I would agree with the sentiments so far expressed. Joiners can be easier at less expensive hotels. I did have one very embarrassing experience at what was then the 5-star Regent Hotel on Rajadamri (so we are going back quite a few years - it later became the Four Seasons and is now the Anantara). I took a boy back from one of the bars at around 10:00 pm. I knew there were security staff on duty at the lift lobby and managed to get into a lift without being seen. I had forgotten about cctv cameras, though. Within a minute of getting to my room there was a knock on the door. It was security. I did not answer. After various similar episodes, the phone rang. And then rang again. I refused to answer. The end result was a less than satisfactory tryst. I also agree with @reader that the Montien definitely had a no joiner policy.
I have written before about a conversation I had with the Assistant GM of the Mandarin Oriental hotel - again quite some time ago. We were having drinks in the lobby when a young man entered with what was obviously a bar girl. Before they even got to the lifts, a security officer approached them. End result? The girl had to leave. No argument about it. I asked my colleague what the hotel's policy was. No joiners after a certain time as it was against the law for hotels to accommodate non-guests who were not registered. But if the guest brought a girl or a boy who was dressed in line with the hotel policies - i.e. pleasantly casual but no tank tops, no  flip flops etc. - then they would be given the benefit of the doubt and allowed upstairs.
I have heard that there is no problem with joiners at Le Meridien on Suriwong. When I spent my one test&go night there last week, I saw no security in the lobby at all and the lift area can not be seen from the front desk. At two mid-price hotels in that area there used to be desks by the lift lobby to get extra payments for guests taking joiners to their rooms. Not sure what the situation is now. Checking hotel policies in advance as suggested by @z909 is a good idea, although given the government's overnight rule, I still think it is best to book a double room and then have your boy du jour register.
Chiang Mai
There is certainly a lot to see in and around Chiang Mai. From @musgrave's post, though, it seems Chiang Mai's gay scene continues in the death throes that started more than 5 years ago. We found the bars now situated across from Le Meridien boring. The money boys are more interested in the locals who frequent them or in their cell phones than new customers. There are certainly other gay massage spas in addition to Classic. The two gay saunas seem to be doing quite good business. Club One-Seven is located near the southern outskirts on the river across from the huge Holiday Inn Hotel. The older House of Male in the north west of the city always  seems to have some students from the nearby University, but with the Songkran holiday about to start I suspect there may not be many until term time starts again. This site is worth checking - 
https://www.gay-in-chiangmai.com

Chiang Mai is showing signs of life again particularly with the approach of Songkran - Adams Apple remains closed and Pandee Bar, Secrets Bar have ceased operation. In the night bazaar area Orion Bar and Ram Bar are open and provided cabaret shows. There are several boy host bars Pose, Maya, Route 181 which really don't kick off till 11.00 pm - staff at these venues can be invited to drink with you at a fee per hour - post work arrangements can be negotiated individually. Barrocco karaoke works similarly. Most of these venues have Facebook pages.
The gay Apps used here include Hornet, Grindr and Blue'd and there are plenty of members. The authenticity of some profiles is questionable.
Chiang Mai old city is a great place to wander around and the Saturday and Sunday walking street markets are worth a visit. 
Food is a great attraction here as is coffee - some excellent coffee locations. You have to try khao Soy crispy fried noodles atop a coconut curry broth with chicken and yellow noodles.
Classic House massage is close to the old city and remains one of the steadfast gay massage places. Woody at Classic House also does tours and he is a really good guy - a day tour costs around 2500 - 3000 thb
Some really nice places to visit outside Chiang Mai - Doi Inthanon, Mon Chaem, Mae Khompong, Chiang Rai (overnight is best). However there is haze around at present which limits views. The large mountain Doi Suthep overlooks Chiang Mai and is a nice spot on a clear day.
At present one of the fads is going to a watercourse , sitting on a chair in the water and eating and drinking and getting wet of course. Lots of people go to the lake Huay Taeng Tao to eat and spread out in the bamboo huts overlooking the water.
 
 
 
 
 

Any details much appreciated. I will be in BKK in May.