I tried to define macros in init_custom.ftl but file appeared to be ignored probably due to freemarker namespacing issues.
Tried it on both 6.0.6 and new nightly build of 6.1 released 2011-10-10.
I also tried explicit import of file into namespace and then using that namespace to call macro. That also didn't work.
Also try to make sure that variable overrides in that file work too.
I tried following and it didn't in 6.0.6
<#assign company_name = community_name />
<#assign company_url = community_default_url />