Comment 6 for bug 1642669

"The file does not exist because /usr/lib is hidden by a mount, it looks like. And we can't just point at the copy inside the snap because it lacks the setuid bits that the system copy has."

The /usr/lib being hidden should only be a problem on classic and I guess you are trying to leverage the already installed policykit from classic. For this part of the problem I might point out that snaps can ship setuid binaries (eg, the core snap does today), it is just that they trigger a manual review in the store. It is therefore possible for you to ship /usr/lib/policykit-1/polkit-agent-helper-1 somewhere in your snap with the setuid bits set, and then the review tools can be updated to allow this for your snap. It isn't terribly pretty, but it is possible.

For all snaps, you will have to consider my previous comments (along with Gustavo's input) since policykit won't be available. (You can use the setuid trick with all snaps too).