Hi Clockwork (orange?....couldn't help myself)
I think it is actually easier to build the gantry with a couple of moving mirrors.
If you are going to encase the whole thing and blow some fans inside to use a positive air pressure for the case then you will need a big exit hole for the air to exit otherwise the fans will cavitate and you will have no real air flow to speak of. Try putting a piece of newspaper over a fan with a small hole the size of your thumb in it and see how much air comes out. Not much. But I guess you have already figured that out anyway. If the tube isn't moving then there won't be any real problem with it except getting the water to flow in the right direction. (heat rises etc....)
I tried the small compressor for air brushes and unless you have a hole below the lens that is about 0.010" in size you will get too rapid a drop in pressure and it will be of no benefit. You get pressure but very little volume from them and that is what you also need, not just pressure. Try a high volume electromagnetic aquarium air pump. Much better but you will still only get a few pounds of pressure from it but it will last for years.
If you have too much pressure then you risk actually blowing the material away you are trying to cut and even if you hold it down with several magnets then the forced air pressure will get under the job and make it flap about like a fish out of water.
If you have a moving table then you are restricting yourself to light and small (not tall) objects because you have to move the whole job around with the table as well. If all you want to cut is paper and cardboard then all will be fine but if you want to engrave a 25lb slab of granite then you will have to reduce your speed to overcome the inertia of the whole job plus the table weight and then you will have an all night job on your hands which will get very boring VERY quickly.
All the manufacturers build XY tables and they have the resources to invest in research and development so I think they got it right years ago. I always say "don't reinvent the wheel" but the final choice is up to you so don't forget to take pictures or/and video of the build. They're always interesting for young and old.
Rich.
I am not completely useless.......I can always serve as a BAD example.