I think if you research the OSHA regulations deep enough, you'll find more than one answer to your question. It appears to depend on just what type of work area you're talking about.
For example, 1910 Subpart S (1910.303 specifically) states that, for 600 volts or less, "The minimum headroom of working spaces about service equipment, switchboards, panel-boards, or motor control centers shall be 6 feet 3 inches." In the case where energized parts over 600 volts are exposed, this requirement increases to 6'-6".
On the other hand, under 1910.179, "Overhead and Gantry Cranes", the statement is made "Where footwalks are located in no case shall less than 48 inches of headroom be provided." This seems rather tight and I'm assuming it would only apply for cranes; not any permanent platform in a building.
As far as "Means of Egres", I would agree that the 7'-6" headroom applies. Just what actually is included within a building as a "Means of Egres" is somewhat vague. If you look at OSHA's definition, "A means of egress is a continuous and unobstructed way of exit travel from any point in a building..." and includes "intervening room spaces, doorways, hallways, corridors, passageways, balconies, ramps, stairs, enclosures, lobbies, escalators, horizontal exits, courts, and yards."
Hope this helps.