Subsetting, performing band maths and then stitching the subsets together

Dear users and developers,

In my workflow I need to subset one scene into smaller rasters, perform band maths and the stitch them together again before I perform the geometry correction. Mosaicing works only after geometry correction and with a defined projection, which I would like to avoid in order to keep the rectangular raster.

What I haven’t figure out is how to stitch them together before geometry correction. Is there a way to do this in SNAP or snappy?

Thank you in advance

It might help to indicate why you need to use subsets. Would it work to define masks to select the subsets and then write bandmaths expressions that only touch the data for a particular mask?