Mechanism for hidden/magic settings for a custom connector?

[redec]

I'm writing a custom data source connector, and I'd like to have some hidden mechanism to do magic settings.  Specifically I would like to selectively override the remote URL my extension connects to.  By default my extension would connect to the main production environment, but I'd like a way to configure it (for dev/testing/demos/whatever) to point at one of my staging/preprod environments without needing to do a custom build of the extension for each environment.  A typical enduser should never see anything (I don't want to be outright prompting the user etc).  I was thinking I could just have a magic file on the harddrive which specifies the url, but it seems like any call to File.Contents results in an UnpermittedResourceAccessException, even when running as administrator....I don't really see anything in the framework which could allow for reading envvars or registry keys or anything like that....at this point I'm thinking I could have a sidecar application that opens a port on localhost which my extension will attempt to connect to get dev settings, but that kinda sucks and I'm thinking there must be a better option that I'm missing...anyone have any thoughts/ideas?

[redec]

Is there a way call an enternal query from an extension? (like the approach taken here https://stackoverflow.com/questions/52878319/load-data-into-power-bi-from-relative-path) ....cuz that would be perfect, except I can't get it to work.  I would have expected Expression.Evaluate("mySettingQuery") to do the trick, but I get "The name mySettingQuery does not exist in the current context".  Heck I can't even figure out how to make this work even if I was prompting the user for it directly - even if it's passed into my main datasource entry point, afaict there's no way to actually pass it through to my authentication StartLogin method, and I can't do anything with side-effects (like set a global var).  It seems like I must be missing something fairly fundamental (I'm very new to this)....is this not a thing people do?  Or do people just resort to having multiple builds for different environments/settings?