invoke the exported compiler [...] passing a text input representing a program written on my DSL
Your idea is sadly inherently flawed. There is no such thing as an MPS "DSL compiler" which takes text as input. In MPS there are generators which transform your DSL into another MPS language, in your case your target language would be BaseLanguage (MPS version of Java). After the transformation, the Java source code is generated as .java files and is automatically compiled as .class files. So yeah, this can be done with an Ant script built in BuildLanguage and called from cmd. But, the generator does NOT take as input text but an AST. The AST is your program "coded" (proper term would be modeled) in MPS.
So what you actually want is a parser (if your language is textual and parseable that is), which has text as input and AST as output. Once you have the AST in any form, you can somehow put it into an MPS model.
Please refer to my other answer where I commented on some portability (basically import, export) in MPS here. I have mentioned (not only) a project I am working on there. It allows to import a language and programs into MPS.
If you don't want to use MPS' IDE at all, but to work with text, it loses the advantage of MPS as a language workbench (LWB) with projectional editor. Maybe you should use another textual LWB (f.e. Xtext) or a parser generator (f.e. ANTLR). If the grammar definitions in parser generators scare you, you could use a model-based parser generator like YAJCo (I have contributed).