well we don't think we can make HL2 hook dlls *unless* we are using non-steam warez version
you just can't specify a replaced game dll other than "bin\server.dll" in liblist.gam. You can use gcfscape to extract the gcf file and take a look at the liblist.gam file.
The only way to replace the game dll will be renaming the game dll, but Steam will just replace it back when it launches.