OK, I'm working on this as (hopefully) the final thing before the next beta.
As mentioned, what Android refers to as 'external storage' includes, but is not limited to, SD cards. A this point, we are leery of, in our V1.0, including hard-coded hacks to directly use the SD cards in contravention of standard Android behaviors. (It may buy everyone grief in the short (and possibly long) haul, and we can certainly revisit this whole issue later.)
(There are also issues with removing content in non-standard, hacky locations when the app is uninstalled.)
So, for now, when the app is installed, if Android reports to the app that 'external storage' is available, we'll store a flag in the app that records that we're using external storage--and content will all be installed to 'external storage.'
Note that this may, or may not, actually end up on the SD card, depending on the OS version and the device manufacturer. For now, we're going to let the Android OS and device sort this out. At least we know we can use this supported API to find the content and even relocate it later, if / when we research or develop a more elaborate, hopefully less hacky, approach.
That's what I think I know. Shouldn't take too long. Famous last words and all.