Common practice is to omit the accents on vowels in all caps situations (in scrolling signs) when space is at an absolute premium, even the Ü gets it elided sometimes (you'll see ARGUELLES and ARGÜELLES for in Madrid transit signage). The Ñ normally gets rendered with the right vertical line slightly lowered to make space, or the entire N made smaller with a flat vertical bar for the tilde. Ocassionally you might see !? instead of ¡¿ on some systems.
I agree with Walter Mitty's comment that, region-depending, ny and nh are combinations you'll see to replace ñ. Those closer to Portugal or Brazil will tend towards nh, all others towards ny.
However, unfortunately, you've said you're stuck with lowercase. While the text will be readable, it's going to look horribly, horribly unprofessional to not use accents and I can't discourage you enough from doing it. I get that you're stuck with what you have, but if you're looking to make this work for multiple languages, I'd highly recommend investing in a system that can handle more characters.