You could use an ItemGroup to specify such files and reference the ItemGroup in the Files parameter. Something like:
<ItemGroup>
<MyFiles Include="*.txt" />
</ItemGroup>
<FindUnderPath Path="$(OutDir)" Files="@(MyFiles)">
<Output TaskParameter="InPath" ItemName="AllTxtFiles" />
</FindUnderPath>
Source: http://msdn.microsoft.com/en-us/library/vstudio/ms164293(v=vs.120).aspx