Couldn't it be done in the age-old tradition of UNIX/Linux GUI frontends to command line apps? It seems like your installer (or the GUI app itself, on first run) could do all the work of downloading the Tarsnap source, compiling it and then simply launch the CLI program to execute the actions.
It'd take some dev work to set it all up, but could be transparent for the user.
On OS X that requires downloading a 2 GB package (Xcode) first to install the compiler and such. Users would rightly think that's overkill for a backup tool.
Yes. Also, installing the command line tools on a pristine system is easy and does not require downloading Xcode. Typing cc in Terminal will pop up a dialog box that will offer to install the CLT.
I don't get what you're saying. Why would you lose verifiability or security? And the hassle would be only for the developer, not the user.
Oh, the installer would obviously check the source archive against a bundled copy of the current Tarsnap GPG key before compiling. I just wasn't detailing implementation details.
It'd take some dev work to set it all up, but could be transparent for the user.