I am aware that strobe lenses must be clear or white, but is text on the lens allowed?
It wouldn’t really make sense for a clear, white strobe to say “FIRE”, so they usually put them on the side of the strobe or on the body of the signal itself. Colored lenses cannot be used for fire protection, but some companies, like System Sensor, have label packs that you can put on the body of your strobe to give it text like “ALERT”, “AGENT”, etc.
Although white lenses are allowed, most manufacturers use clear lenses because they allow the maximum amount of light to pass through. Anything that absorbs some of the flash has to be made up for by making the flash more powerful. That takes more electrical power. ADA has very specific guidelines for the amount of light that should be seen at various angles. See the chart below. Lettering on the lens would block some of the light which makes achieving the required dispersion pattern more complicated.
Chart is from this web page>
http://www.access-board.gov/guidelines-and-standards/buildings-and-sites/about-the-ada-standards/background/adaag-review-advisory-committee-report/chapter-7