We have not defined a schema for .bld files. The basic structure is pretty simple:
<?xml version='1.0'?>
<project version='5'>
<!-- optional comment-->
<comment>comment</comment>
<!-- 0 to n steps -->
<step action='Group' type='0'>
<name>Project steps</name>
<!-- more optional properties specific to action -->
</step>
<!-- 0 or more macros... -->
<macro>
<name>MacroName</name>
<value>the value</value>
</macro>
</project>
However, there can be any number of additional elements (one for each property) within a step element, and each action type has different properties/elements specific to that action type (macros can have a few additional optional elements as well). I'm not sure if SQLXML would accept a matching open-ended schema or not.
You may want to run your project file through an XSL transformation first to filter on the specific step actions + properties you're interested in, then define a schema for the output document and use those for importing into SQL Server.