I was originally worried that having to work out the deps could itself be annoying for users. I'll merge it, but do you think there's a nice way to somehow make it toggleable, or if that's a good idea?

It's difficult, cause pip just kinda sucks. :( No API (well, at least no stable documented API, I'm sure it's possible to just import stuff), you're stuck with the commands they give you. Ubuntu also ships an OLD version of pip which has additional issues with the --target parameter, and this is likely what many people will have. If it would simply look in the right place to determine if a dep is present it would work.

I tried a number of things, including installing pip into hostpython and invoking it via targetpython etc. I even have a install_into_targetpython option in my code to allow for host-only recipes just for this. (I'll probably make a PR with it at some point, but it's unused now. Could be useful if we wanted to install cython in hostpython, which I think is a pretty damn good idea.)