I'm quite sure Japan was not able to fly a captured B-29, if you mean that by "evaluate".
As for crash-landing B-29 leaving wrecks in condition good enough to give a good idea of what they were, I don't know but the link below may help:
http://www.powresearch.jp/en/archive/pilot/index.html
The links inside this page will point to 6 PDF reports listing Allied losses (USN, USAAF and FAA) over Japan (divided in 6 areas) with the fate of the crew.
I have found them yesterday, and have not finished reading, but for many losses there are details.
Hope this helps
Laurent