Some custom widget magic might be able to help you out. Depending on what you want to be configurable exactly, you should be able to model some settings in a domain model and then read them from the client via a custom widget. Once you have the settings there, you can do a lot of funky stuff (refresh at will, change the background color to green etc)
Although dynamically changing stuff might influence stability and performance. Furthermore it dramatically decreases maintainability (for example it is hard to say whether something in a new version of the client has changed in such a way that it breaks your widget)
So in the end the question is often not, can it be done, but, is it worth the effort. (and do I have a spare JS programmer ;))
The parameters in the modeler can't be changed. You have to use full development platforms (Visual Studio etc) instead of Mendix if you really want this.... You can create an entity with settings, in your microflows you can test on values of the settings and open special forms, execute different microflows.
Thanks for the reactions. @Chris: Yes your option is a possibility, but not what i'm looking for :-) (thanks anyway!). Reason: it is not as flexible as having real parameters (and maintenance is (a lot) more (more forms, microflows etc).
Visual studio... No, I like the concept of Model Driven Development. I wonder if I'm the only one who feels the need for defining parameters?
@ Achiel: Can you explain this a bit more? Do you mean that it is (theoretically) possible to program a widget for this?