I think this is not that academic way Guix is intended for. But such a feature will improve user experience riding him from long time compilation.
Guix also may solve the problem of absent substitutions the next way.
Guix developers can create user interface for sending requests to default substitution server to compile those packages users want right now.
As I know, hydra compiles all the packages for all the systems without priorities.
So users can address to hydra their requests to compile some packages for their system type (and environment). So hydra will stop its usual way of things (that actually has the lowest priority) and start to compile packages on user's behalf.
This way user can send a request, wait for 30-120 minutes and install it or update to having substitution built on hydra.
Reading some discussions about OS, I can say this is the first user want to have, saying: 'Computing freedom is imposed by Guix, but that ugly compilation process is what does not give me to start using it'.
Please, could you try to satisfy new users by speed up packages installation?