I cannot speak with authority since I do not yet use the compiler for Holtek, but I would be surprised if any startup RAM usage is a permanent allocation and not available to the application after startup. The code space that runtime startup uses can include a loop to clear .bss and code plus initializer values for .data. Unless you've got a "huge" amount of variables to initialize, I wouldn't expect .data initialization to be "huge". I could be wrong.

Of course you could determine most of this yourself by downloading the free but time-limited demo version.