Why does it have to be at a particular location?

Anyway, if you want it to be accessible to 'C', you must follow the C51 conventions of byte ordering, etc.

Then all you need to do is define it as PUBLIC in your assembler, and have a corresponing 'extern' declaration in 'C'
'C' neither knows nor cares that it actually came from an assembler PUBLIC!

Alternatively, write a little "translation" function in assembler. This must follow the C51 calling & return-value conventions, but internally (in assembler) can do whatever bizarre actions are necessary to access your arcane assembler data!