As I've noted: I've forgot that you are one of the very few Bitcoin developers with regular access to big-endian hardware. I'm thinking that you will continue to be the only one developer who can test picocoin on the big-endian architecture. The mojibake-endian problem will continue to badly affect the future contributors/users of your library if you aren't going to use your own medicine (that is "sparse" and "__le32")
FWIW while I've never tried doing it QEMU can apparently emulate the big-endian PowerPC architecture.
Here is one guys notes about getting it working from two years ago, specifically to test endian issues.