[StavrosK]

How can you check that the closed-source process won't ever read the key from the disk without elaborate contortions?

[mariusblaesing]

What should it do with it? It can't send it anywhere: Block network access for that process if you don't trust it.

Key won't be accessible on disk anyway, only in RAM during crypto and will be destroyed immediately afterwards.

[StavrosK]

You can make it so that it will only store the key in RAM, but then you'd have to enter the key every time it launches, making automatic backups impossible.

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.