Okay, some info and some ideas to get you started, maybe?
Let's think about the outside wall. It may not be as hard as many assume. The standard wood wall is really pretty easy as it is just a very simple sandwich of inner wall covering like sheetrock on top of 2X4? studs and then the outside covering. So two big things before rejecting that idea. One is what the outside wall is covered with and how it works. Simple wood is the easy thing to find as it cuts so well. A bit harder is if it is cement board and worst might be brick. Can be done but harder to cut brick well.
Maybe works, maybe not?
Any chance of it being over an unfinished basement or crawlspace? Those are nice for making it easy.
Second on the outside question is where do you go once outside. Drains to flowerbeds are nice as the tank water is so rich in nutrients that work well for garden plants. If you don't want to dump out where it is a problem to walk, etc., considered going out and then back in to a drain?
One biggie problem with outside plumbing can be cold weather. How cold does it get in your area? Pipes can be insulated but it can be trouble.
Some things I have done vary from real easy to very difficult. Toughest was a full blown auto change using a reserve water barrels, siphon from tank to tank to drain off overflow and using a pump in the reserve to force water to the tanks. With float switches and solenoids to turn the water supply on to refill the barrel, it was fully auto and needed no care as the water was only treated with chlorine. The chlorine just gassed off before then next water change on timers. Worked great except the siphons which were a constant problem!
I now use a barrel and have a faucet in the former bedroom which we have made playroom for hobbies. Turn on the faucet to fill the barrel while adding dechlor, let the water warm to room temp and then on change day, I pump it to the tank using plastic tubing run across the floor and/or up the hall, depending on which tanks. I have drains through the outside walls for tanks in the hobby room as well as one out the other wall of the dining room. In this part of the world, water for plants is almost always good so I take care of a couple beds on change day.
Any chance of being on a crawlspace or is the guest house on a slab? Still lots of options that you can work through. The drains are usually easier as it does just take punching out through a wall and letting it run downhill. When you get down to it, houses are often not that tough and you can kick your way through many walls. Saws and drills make neater holes, though.
