It's not what you want to hear, but the answer is no, by default you are limited to the /testcontainer
or /testmetadata
options for specifying which tests to run.
You could easily run MSTest multiple times for each DLL in your project which contains tests. You could even run a process that analyzes your DLLs for tests and build up a list of DLLs which contain tests and then run a script that calls mstest for each of those DLLs.