There is no official/guaranteed/documented way of doing this in terms of support contracts, but have no fear, it works like a charm to use the eZ Find 2.1 Solr binaries with 2.0.
Actually, can you tell me what kind of customisations you did that hamper a drop in replacement of eZ Find 2.1?
The upgrade to eZ Find 2.2 will be a tad harder (you will need to re-index your sites as the Solr schema will have some changes and the way eZ Find will deal with facets vs searching)