For those on "local" terms your SALARY is exactly the same as the expat that you joined with. The difference occurs with "allowances".
The "expat" receives housing allowance due to the cost of housing in HK comparable with the country they have been recruited from. The fact that the company allows some people to use that for a mortgage is a "company" decision and purely financial, it is cheaper for them in the long run and "ties" the recipients to the company.
The education allowance is given as the government does not provide free "english" based education, whereas the "local speaking" can receive free education.
So you are NOT being discriminated against as you receive the same salary, flying allowances, travel and right choice benefits, depending upon your date of joining. Local Captain's receive a "special" allowance for being at the appropriate level of management, the same as other non-flying "local" managers.
It is like those who do not have children asking for the education allowance because other people are receiving it so they should, they are getting paid more than me.
If I was at my home base then I would not receive, quite rightly, housing or education allowance. The same argument put forward against extensions can be used, you knew what you were signing up for - live with it.