As I understand it, it maybe simpler than you think. Assuming your LR software, any Catalogs and their associated preview folders are on the SSD the performance penalty for having the image files stored externally is minimal provided automatic write changes to XMP is turned off. At a suitable time you can write the XMP changes to your external image files instead of shifting image files around. The initial import of your photographs will not be any faster but searching, editing, exporting etc should reflect most of the speed improvement the SSD imparts and you will not need to shift folders around.
Here is a
LINK to an article that may be useful but it does not answer your question specifically.