If this software solves a problem you are really having, nothing would stop you.
I'm really liking some of the things Microsoft is doing with Power Query, but I don't like how it is (afaik) only callable from Excel or PowerBI online. I'd like similar capability, but more open, and could be called via scripting, from SQLCLR, etc.
Another big hitch: Microsoft has not published proper API's for manipulating PowerPivot models in Excel, and I don't think they intend to - I've heard one 3rd party has reverse engineered the API's (you can decompile the .Net binaries, but I haven't had the time to look at it yet).
To expand on my question, I'd heard (from Rob 'PowerPivotPro' Collie, former product manager on the project IIRC) that the core had been written in 'unmanaged code' (probably C++?), so I believe reverse engineering it would be a significantly larger effort than just opening some of its DLLs in say DotPeek, at least from as far as I've been able to tell.
By writing to the model, you mean programmatically adding new measures or the like?
My interest is in programmatically querying models using DAX, though to this end I'd also look to look in the direction of Microsoft's DirectQuery mode in SQL Server which supposedly did DAX-to-SQL conversion.
If one could use such a conversion plus MDX to start querying models on an Apache Spark cluster through pivot table/chart interfaces...