Code snippets don't really help me with this as I write/add a lot of LINQ extension methods.

Joe (the author) was kind enough to respond to an email I sent on this topic. His response was, "Create a code snippet and LINQPad will pick it up.... If it conflicts with an inbuilt snippet, your snippet will win."

I have been able to do this with snippets that I have explicitly selected from the list, but not for the ones that are triggered with autocomplete (like the space after an extension method).

Yes, it would be great to customize this particular behavior. It really bugs me to have a space before parentheses when I'm invoking a function. A space after if, using, while, etc makes sense, but please don't put one after a function. (Interestingly, when you open up a C# Program query, the main method is written *without* the annoying space, viz. void Main().)