to change common properties for all your projects at once, you could import a common project in all of them. Yes, this requires editing all your current projects (although it's not that hard to write a script listing all .csproj/... files recusrively and adding/removing a couple of lines), but if you have to make changes again in the future it will make things a lot easier.
For example, in all your projects use this (as last line before the end tag):
<Import Project="..\templates\mydefaults.targets/>
And in mydefaults.targets, for example:
<Import Project="mypaths.targets"/>
<PropertyGroup>
<OutputPath>$(MyCommonBinDir)\(Configuration)$(PlatForm)\</OutputPath>
<BaseIntermediateOutputPath>$(MyTempDir)\$(AssemblyName)\</BaseIntermediateOutputPath>
</PropertyGroup>
I like to keep the mypaths.target seperate: it defines MyCommonBinDir and MyTempDir, so if I want to build a branch of my original source tree I only have to change that file to set the output paths for all projects at once.