I think you could avoid a lot of the network traffic, sandbox issues, and cargo issues by having a curated set of crates that could be used in the playpen, and having them available pre-compiled on the playpen machine.
There are a large number of crates that just wouldn’t make sense due to sandbox restrictions, the platform, the nature of the crate (who’s going to be running GTK apps in the playpen?), and similar issues. You don’t really want to open up a large amount more surface for downloading, compiling, running arbitrary build scripts, and so on from potentially arbitrary crates from crates.io.
So just pick a set of popular and fundamental crates that make sense to use in the playpen, have them available in the search path, and you get to use only the ones that are available there.