Does SourceForge support using /etc/aliases for local mail delivery?

No, SourceForge uses the James SMTP server, which does not use the /etc/aliases file.

To enable local mail aliases, you will need to configure user mapping in the XMLVirtualUserTable in the /usr/local/sourceforge/runtime/james/apps/james/SAR-INF/config.xml file.

Note: Please note that while the James SMTP server is used as part of SourceForge, customizations such as these cannot be supported by CollabNet.
Related information
James Project