Excuse me if this is a silly post, but I have an actual front diff lock indicator on my instrument panel. If you have this, wouldn't you prefer to have that indicate when your ARB front locker is engaged instead of the "replace air filter" indicator?
2000 and later land cruisers have VSC and ATRAC so the indicators are used for those functions. Take a look at my post here http://tlcfaq.com/main/2012/05/factory-switch-to-control-air-lockers-part-2/, scroll to img 3135 (mid page) and you can see the back of the instrument console closeup. The lights that say VSC and VSC off are where your rear locker light is.
The empty slot in that photo is where i installed the indicator bulb and it happens to be the clogged air filter position. In my opinion that indicator seems like a a decent representation of a compressor function. Via the fog light switch installed the compressor can be activated separately for airing up tires etc. In your case you have an electric locker so no compressor needed.