How to retrieve InChI key for KEGG compound?
I'm not aware of a functionality in the KEGG service that would allow you to do that directly. I believe your solution (using e.g. ChEBI) is the correct one. 

You may have other useful service such as unichem, chemspider (you would need a login though for that one) could help as well. 

Disclaimer: I'm the bioservices main author