[plt-scheme] Is there any way to disable object moving in GC temporarily?

Hi,

For some reason, I can't use the MZ_GC_DECL_REG, MZ_GC_VAR_IN_REG and
MZ_GC_REG related macros provided by mzscheme sometimes. I'm just
wondering is there any function to prevent some pointer pointed object
from being moved. Just like the scheme_dont_gc_ptr function to prevent
an object from being collected. Or is it possible to write such a
helper function?

Thanks!

-- 
pluskid