Set Minimum Quality Level
I like how the mode option works for setting a maximum quality level, e.g. --mode=better. I would like a similarly easy way to set a minimum quality level, i.e. without manually specifying all of the modes I wish to target.

My approach would be to have an option such as --mode-exclude which can be used in conjunction with --mode, e.g. --mode=better --mode-exclude=worse. Hence, Get_Iplayer would only target those modes which are covered in better, but not in worse. The option could also be used to exclude certain modes directly for those people who don't like using quality levels, which would still be easier than specifying a long list of modes to include in some circumstances.
Sorry, but it's a no from me. Few people would use this, and I'm not willing to support it. As you implied, you can configure whatever recording modes you like to achieve the same effect. This list would not be particularly long (there are only 17 possible entries and you would only use some of them), and you would only have to do it once. More generally though, if you aren't getting the best available quality (subject to your settings) virtually every time, then you likely have bigger problems upstream.