According to the following passage from the SLA it may be legal to use XPostFacto to install OS X on a unsupported computer if the portion of the code that XPostFact modifies or replaces is open source software and the modified OS is still ran on an Apple-labeled computer. Check!
B. Certain components of the Apple Software, and third party open source programs included with the Apple Software, have been or may be made available by Apple on its Open Source web site (
http://www.opensource.apple.com/) (collectively the "Open-Sourced Components"). You may modify or replace only these Open-Sourced Components; provided that: (i) the resultant modified Apple Software is used, in place of the unmodified Apple Software, on a single Apple-labeled computer; and (ii) you otherwise comply with the terms of this License and any applicable licensing terms governing use of the Open-Sourced Components. Apple is not obligated to provide any maintenance, technical or other support for the resultant modified Apple Software.