Hullo, there is no difference with orientation - a landscape machine is just a portrait machine that you can only use the top half of!

The main problem is the one you've mentioned - getting adequate drop for the counterweights. You can use a pulley systems to get a bit more drop if necessary (http://www.polargraph.co.uk/2014/10/huge-portland-design-week-project/). The second problem is margins - there's always a margin at the top of a machine where you'll struggle to get the pen to move to reliably - given the catastrophic nature of dropping steps during a drawing, it's something to be afraid of.

The actual margin is a function of the width of the machine and the holding torque of the motors - so the wider the machine is, the larger this top margin is, and the smaller your actual drawable area is. If you had a machine that was 8m wide, you wouldn't be able to use the top 800mm of it, which if it was only 1500mm high, that'd be a problem.

happy scribbling 🙂
sn

I've avoided liquid ink pens, but have found other kinds to be surprisingly usable horizontally. If there's a felt cartridge inside, it seems to work ok. Sharpies, and most fibretips.

The bigger problem is volatility. Most large markers that lay down enough ink to be bold at larger scales are strongly spirit-based, and they dry up pretty swiftly with their caps off. Whether it's ink running out, or just solvent evaporating, I'm not sure.

I was recently shown this delight:
http://justinwilcott.com/gocupi-large-canvas-and-paint-gondola/
which is a chunky paint-marker type gondola.

sn