I got an error when I try to push big file (greater than 50KB). I think you can split a big file and push it to datastore. (Use ListProperty with Blob, instead of Blob)
I didn't check this, but you maybe want to try:
You can split bundles by -r flags with hg push.
（I find this at http://hg-repos.appspot.com/static/faq.html）