Boldshare matches your feature list exactly. Client code will be open source. Beta in November:
https://boldshare.com/
On the link you provided I found the following:
"Parts of the programm relevant for security and networking actions will be open source."
"Parts of the program" might not do it for everyone (me included). Will anyone be able to audit the full source code of the client and compile it themselves, like e.g. tarsnap? Otherwise I fear the usual trust issues arise.
If I missed some information at your linked site which says the entire client will be open source, then please correct me.
Updated the FAQ: https://boldshare.com/?lang=en#faq
I read the updated FAQ and saw the note about blocking network I/O for the closed source process. If the closed source process in your client were locked down in such a way that it was only ever invoked from the open source process and had no way of doing any I/O whatsoever (network or otherwise) except with the open source process (where what goes in and comes out can be audited), then I'd agree this sounds quite interesting.
I'll be waiting as well, and sincerely hope you deliver something nice!
Key won't be accessible on disk anyway, only in RAM during crypto and will be destroyed immediately afterwards.
Although, I agree, if you only allow the open source network access, AND can ensure that the key won't somehow be smuggled in the data the closed source process sends, you're probably fine.