Yup. I currently have several installs (WW1, RAF, Luftwaffe, Korea, Pacific). I wanted the option of setting up campaigns where a brit pilot can have a career starting in France in 1940, go through the BoB, then to the middle east, Malta, Italy, back through France and Germany. For example, 263 Sqn starts in Norway with Gladiators, moves to Scotland with Hurricanes, then to the south of England, then SW England with Whirlwinds, then get Typhoons and goes to France and into Germany with 2nd TAF. Same for the Luftwaffe; a career that takes in the early days in the West, then the dreaded Eastern front, then the defence of the reich.... All these have MB directly linked to them as you suggested. The game itself seems to cope with large airbase.dat lists, so I may just have an alternate .dat list that I load for building the mssions, which I don't really do that much of anyway.
A lot of this started because about half the campaign files I download don't work for me, and I can't edit them in MB because they always come up as 'corrupt files' (not to mention the duplication of airbase id numbers). Now that I know about the airbase limit I may have another look, just in case that was a cause; looking for airbases that don't exist.

Thanks again for the help.

Piers