Thanks BurrMan and to everyone who replied.

Here is what I just tried.

1. Open obj in meshlab
2. Go to Filters -> Remeshing, Simplification and Reconstruction -> Simplification: Quadric Edge Collapse Decimation
3. Enter target of 1000 faces
4. Export obj
5. Use Michael's obj23dm converter, choose closed polygon for each face
6. Load 3DM into MoI
7. Select everything
8. Choose Construct -> Planar
9. Save as 3DM
10. Open desired part in Alibre Design
11. Import 3DM surface
12. Trim model using surface
13. Move surface
14. Trim model using surface (reversed)
15. Export as STEP
16. Load STEP into MoI
17. Delete surface

Alibre Design is ACIS based and doesn't like triangle-based geometry but it seems to cope OK with slicing an ACIS object with a triangular surface.

That's a lot of messing around, would be nice if MoI had this trimming function as well.

Andy