Spaced paths, msbuild, and psake - powershell

Question

Related question here.
This works properly for compiling an mvc3 application.
task Compile
{
$config = $script:siteConfig.config
exec { & "C:\Windows\Microsoft.NET\Framework\v4.0.30319\msbuild.exe" $webproject_path `
/p:Configuration=$config /p:WebProjectOutputDir="$publish_dir" `
/p:Outdir="$out_dir" /p:CleanWebProjectOutputDir=False `
/T:_WPPCopyWebApplication /T:ResolveReferences /verbosity:quiet /nologo }
}
All of those path variables are script properties. However, when spaces are introduced in those calculated paths (e.g. the project is moved from C:\Projects\ to C:\Users\ASDFG1\Documents\Visual Studio 2010\Projects) msbuild thinks there's multiple project files. This makes sense but I have to be missing something, getting a parsed variable into quotes shouldn't be this hard.
Variations tried
exec { Invoke-Expression "& C:\Windows\Microsoft.NET\Framework\v4.0.30319\msbuild.exe '$webproject_path' /p:Configuration=$config /p:WebProjectOutputDir='$publish_dir' /p:Outdir='$out_dir' /p:CleanWebProjectOutputDir=False /T:_WPPCopyWebApplication /T:ResolveReferences /verbosity:quiet /nologo" }
exec { C:\Windows\Microsoft.NET\Framework\v4.0.30319\msbuild.exe "`"$webproject_path`"" `
/p:Configuration=$config /p:WebProjectOutputDir="`"$publish_dir`"" `
/p:Outdir="`"$out_dir`"" /p:CleanWebProjectOutputDir=False `
/T:_WPPCopyWebApplication /T:ResolveReferences /verbosity:quiet /nologo }

Answer 1

Using EchoArgs.exe to reproduce the problem, we see that quotes are not being passed to the executable as desired:
PS> $publish_dir = 'C:\Users\Documents\Visual Studio 2010\Projects'
PS> ./echoargs /p:WebProjectOutputDir="$publish_dir"
Arg 0 is </p:WebProjectOutputDir=C:\Users\Documents\Visual Studio 2010\Projects>
PS> ./echoargs /p:WebProjectOutputDir="`"$publish_dir`""
Arg 0 is </p:WebProjectOutputDir=C:\Users\Documents\Visual>
Arg 1 is <Studio>
Arg 2 is <2010\Projects>
Using the backslash-escaping option from this answer, we can preserve the variable expansion and the enclosing quotes:
PS> ./echoargs /p:WebProjectOutputDir=\`"$publish_dir\`"
Arg 0 is </p:WebProjectOutputDir="C:\Users\Documents\Visual Studio 2010\Projects">
Here, the backticks tell PowerShell to treat the quote characters as literal values, and the backslash tells the call invocation to preserve the quotes.
Alternatively, we could stick with a single level of escaping by evaluating the full argument beforehand, instead of inlining the $publish_dir variable:
PS> $publishArg = '/p:WebProjectOutputDir=\"{0}\"' -f $publish_dir
PS> ./echoargs $publishArg
Arg 0 is </p:WebProjectOutputDir="C:\Users\Documents\Visual Studio 2010\Projects">