The simplest way since only ONE HintPath can be used is to use the all-so-nice Condition attribute like this:
<Reference Include="TheAssembly">
<HintPath Condition="Exists('..\My\Assembly\Path')">..\My\Assembly\Path\TheAssembly.dll</HintPath>
<HintPath Condition="Exists('..\..\My\Assembly\Path')">..\..\My\Assembly\Path\TheAssembly.dll</HintPath>
<HintPath Condition="Exists('..\..\..\My\Assembly\Path')">..\..\..\My\Assembly\Path\TheAssembly.dll</HintPath>
<HintPath Condition="Exists('..\..\..\..\My\Assembly\Path')">..\..\..\..\My\Assembly\Path\TheAssembly.dll</HintPath>
<HintPath Condition="Exists('..\..\..\..\..\My\Assembly\Path')">..\..\..\..\..\My\Assembly\Path\TheAssembly.dll</HintPath>
<HintPath Condition="Exists('..\..\..\..\..\..\My\Assembly\Path')">..\..\..\..\..\..\My\Assembly\Path\TheAssembly.dll</HintPath>
<HintPath Condition="Exists('..\..\..\..\..\..\..\My\Assembly\Path')">..\..\..\..\..\..\..\My\Assembly\Path\TheAssembly.dll</HintPath>
etc...
</Reference>
So the answer to the question would be this:
<Reference Include="assembly">
<HintPath Condition="Exists('..\..\csharp\bin')">..\..\csharp\bin\assembly.dll</HintPath>
<HintPath Condition="Exists('..\..\..\..\foo\sdk\csharp\bin')">..\..\..\..\foo\sdk\csharp\bin\assembly.dll</HintPath>
</Reference>
If multiple conditions matches, the last one will be used.