" My architect seems confused by this idea. "

Me too.

I would cap off the water lines and the sewer line, and forget about the sink. Guests should have access to a bathroom, where they can do all that before going to bed, or when getting up in the morning.

I stayed with a friend in Yorkshire whose Victorian townhouse had been a boarding house in the 1920s and 30s. There was only one bathroom per floor, but every bedroom had a sink. It was so convenient! She has a large family and it was so nice to be in and out of the bathroom as quickly as possible. Teeth brushing, cup of water, combing down a stray hair could all be done in the bedroo..

Odd, but not a bad idea if the water is already there and there's space.

There are plenty of old homes with sinks built into the bedroom. Do the guests have their own bathroom or is it a bathroom shared with others? If they have their own, then don't put a sink here. If it is shared, then it is a fine source for water for taking pills or brushing teeth. Remember, if you have a sink you have to also have space to set down a tooth brush and glass.

What size is the room?

Are you thinking of a small vanity or will it be wall hung and if hung, will the exposed pipes be seen?

By corner, do you mean you want the sink on a diagonal?

Does your style lean vintage?

Funny, the older house I'm in now had a kitchenette upstairs. The previous owner did something similar where he turned that into a large vanity with a sink for his two daughters. It freed up the upstairs bathroom - both in terms of people not occupying the bathroom putting on makeup and such - and also not having all that stuff in a smallish bathroom. My sister also did this in her older home (1 bathroom upstairs) she put a mirror, counterspace, sink, lighting in her bedroom that shared a wall (plumbing) with the bathroom.

If you have a shared bathroom - having a sink in the guest bedroom means they don't need to leave much of their "stuff" (toothbrush, shaving stuff(guys) medication, hair dryer makeup, etc. in the shared bathroom - which can be awkward for guests. The photos above are charming, but I'd want some counter space, some storage, as well as a usable mirror and light. I don't know how much room you have, but you can build an a small alcove for the vanity to help separate it from the rest of the bedroom.

Show us the layout of the room and maybe an as-is photo.

Your architect's Google must be broken...

Here's a link to"coffee station in bedroom". Plenty of ideas here on Houzz and pinterest. Typically it's in a master bedroom, and here's an elaborate set up. Something simple could be lovely for early rising guests, especially if you're a night owl.