Well, that's embarrassing. After writing this long post it just started working w/o me doing anything... no idea why it only offered me AZW3 the dozen or so times that I tried before... :/ So, now I have a new question, which is probably a big stretch, and probably not possible...
Is there any way to remotely trigger Calibre to do a conversion on the fly when making an OPDS request?
~~~~~ OLD QUESTION ~~~~~
Not sure if this is a Calibre question, KOReader question, or limitation of the OPDS protocol, but thought that I'd start here.
When connecting to the Calibre OPDS server from KOReader and select a book from my library that has more than one format, I'm not offered an option to select the format. The book that I want to download now was purchased from Amazon, so it's in azw3 format, but KOReader apparently doesn't support that, so I converted to epub, but the OPDS interface only offers me the single file format to download.
- Is this a limitation in the KOReader UI? Is there some way to get it to prompt which format I want?
- Is this a limitation in Calibre? Can I configure which file format to default to?
- Is this a limitation of the OPDS protocol, only supporting a single file? If so, well, hopefully there's a good answer to one of the previous two bullets.

Thanks!