Saying 'Best read in Landscape' is fine but that doesn't solve the problem of what happens when you're in Portrait.
That is, the lines still have to work both ways, Portrait and Landscape, so you're back to figuring out how to format a line that's too long.
Note that you have some flexibility in epub and KF8, but not in old mobi.
If they don't want indents, perhaps you can make a small margin-top, say 0.3em, between lines. Then lines that wrap won't have this margin-top and be closer together. That way, lines that wrap will be grouped and be visibly distinguishable from separate lines.