So far, I haven't had any luck getting the linux E@H app to work with the FreeBSD native boinc client. I'm not yet sure whether the problem is the project's update to the albert client, which seems to have also ruled out the use of app_info.xml; or if the difficulty is caused by not tweaking all of the XML properly. The symptom is still the "platform 'i386-portbld-freebsd6.0' not found" message.

So my question, for someone familiar with the core client code, is: If I can get ahold of a linux client, and possibly manually edit some XML files, will the client use different platform names for different projects?

Yes - you are using an "anonymous platform" and you'll need an "app_info.xml" file. You can search on these in the Wiki for more details. I would also suggest you get the Altivec-optimized Mac SETI application from Team MacNN and put it on your Macs - this will give you an example of the xml file needed, how to override the project's names, etc. Plus it'll double your SETI throughput on G4's and G5's. :-)